Section 2.1 – Staffmods
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The first thing I'd like to mention is to not confuse staffmods with automods. Staffmods are added to items and provided "+skills", all possible skills are listed below. Automods are also added to items but are not "+skills" instead they either add poison damage (necro heads), resistances (paladin shields), etc.
If we're talking about any "leafable" staff possible, you can have up to +3 to up to 3 skills. These skills must all fall within a block of four adjacent rows/tiers. That is, you can't have 1st tier skills together with 5th or 6th tier skills and you can't have 2nd tier skills together with 6th tier skills.
-Adeyke
The above quote explains that "tiers" are used to determin whether or not a skill can be successfully spawned on an item that allows staffmods. 1st tier skills must cannot be spawned with 5th or 6th tier skills, 2nd tier skills cannot be spawned with 6th tier skills, and vice-versa. To know what tier a skill belongs to, simply look at what "row" is appears in when you look at the skill page while playing Diablo 2.
All possible skills: (assuming they don't get out of range)
Id charclass skill
6 ama Magic Arrow
7 ama Fire Arrow
8 ama Inner Sight
9 ama Critical Strike
10 ama Jab
11 ama Cold Arrow
12 ama Multiple Shot
13 ama Dodge
14 ama Power Strike
15 ama Poison Javelin
16 ama Exploding Arrow
17 ama Slow Missiles
18 ama Avoid
19 ama Impale
20 ama Lightning Bolt
21 ama Ice Arrow
22 ama Guided Arrow
23 ama Penetrate
24 ama Charged Strike
25 ama Plague Javelin
26 ama Strafe
27 ama Immolation Arrow
28 ama Dopplezon
29 ama Evade
30 ama Fend
31 ama Freezing Arrow
32 ama Valkyrie
33 ama Pierce
34 ama Lightning Strike
35 ama Lightning Fury
36 sor Fire Bolt
37 sor Warmth
38 sor Charged Bolt
39 sor Ice Bolt
40 sor Frozen Armor
41 sor Inferno
42 sor Static Field
43 sor Telekinesis
44 sor Frost Nova
45 sor Ice Blast
46 sor Blaze
47 sor Fire Ball
48 sor Nova
49 sor Lightning
50 sor Shiver Armor
51 sor Fire Wall
52 sor Enchant
53 sor Chain Lightning
54 sor Teleport
55 sor Glacial Spike
56 sor Meteor
57 sor Thunder Storm
58 sor Energy Shield
59 sor Blizzard
60 sor Chilling Armor
61 sor Fire Mastery
62 sor Hydra
63 sor Lightning Mastery
64 sor Frozen Orb
65 sor Cold Mastery
66 nec Amplify Damage
67 nec Teeth
68 nec Bone Armor
69 nec Skeleton Mastery
70 nec Raise Skeleton
71 nec Dim Vision
72 nec Weaken
73 nec Poison Dagger
74 nec Corpse Explosion
75 nec Clay Golem
76 nec Iron Maiden
77 nec Terror
78 nec Bone Wall
79 nec Golem Mastery
80 nec Raise Skeletal Mage
81 nec Confuse
82 nec Life Tap
83 nec Poison Explosion
84 nec Bone Spear
85 nec BloodGolem
86 nec Attract
87 nec Decrepify
** nec Bone Prison
89 nec Summon Resist
90 nec IronGolem
91 nec Lower Resist
92 nec Poison Nova
93 nec Bone Spirit
94 nec FireGolem
95 nec Revive
96 pal Sacrifice
97 pal Smite
98 pal Might
99 pal Prayer
100 pal Resist Fire
101 pal Holy Bolt
102 pal Holy Fire
103 pal Thorns
104 pal Defiance
105 pal Resist Cold
106 pal Zeal
107 pal Charge
108 pal Blessed Aim
109 pal Cleansing
110 pal Resist Lightning
111 pal Vengeance
112 pal Blessed Hammer
113 pal Concentration
114 pal Holy Freeze
115 pal Vigor
116 pal Conversion
117 pal Holy Shield
118 pal Holy Shock
119 pal Sanctuary
120 pal Meditation
121 pal Fist of the Heavens
122 pal Fanaticism
123 pal Conviction
124 pal Redemption
125 pal Salvation
126 bar Bash
127 bar Sword Mastery
128 bar Axe Mastery
129 bar Mace Mastery
130 bar Howl
131 bar Find Potion
132 bar Leap
133 bar Double Swing
134 bar Pole Arm Mastery
135 bar Throwing Mastery
136 bar Spear Mastery
137 bar Taunt
138 bar Shout
139 bar Stun
140 bar Double Throw
141 bar Increased Stamina
142 bar Find Item
143 bar Leap Attack
144 bar Concentrate
145 bar Iron Skin
146 bar Battle Cry
147 bar Frenzy
148 bar Increased Speed
149 bar Battle Orders
150 bar Grim Ward
151 bar Whirlwind
152 bar Berserk
153 bar Natural Resistance
154 bar War Cry
155 bar Battle Command
221 dru Raven
222 dru Plague Poppy
223 dru Wearwolf
224 dru Shape Shifting
225 dru Firestorm
226 dru Oak Sage
227 dru Summon Spirit Wolf
228 dru Wearbear
229 dru Molten Boulder
230 dru Arctic Blast
231 dru Cycle of Life
232 dru Feral Rage
233 dru Maul
234 dru Eruption
235 dru Cyclone Armor
236 dru Heart of Wolverine
237 dru Summon Fenris
238 dru Rabies
239 dru Fire Claws
240 dru Twister
241 dru Vines
242 dru Hunger
243 dru Shock Wave
244 dru Volcano
245 dru Tornado
246 dru Spirit of Barbs
247 dru Summon Grizzly
248 dru Fury
249 dru Armageddon
250 dru Hurricane
251 sin Fire Trauma
252 sin Claw Mastery
253 sin Psychic Hammer
254 sin Tiger Strike
255 sin Dragon Talon
256 sin Shock Field
257 sin Blade Sentinel
258 sin Quickness
259 sin Fists of Fire
260 sin Dragon Claw
261 sin Charged Bolt Sentry
262 sin Wake of Fire Sentry
263 sin Weapon Block
264 sin Cloak of Shadows
265 sin Cobra Strike
266 sin Blade Fury
267 sin Fade
268 sin Shadow Warrior
269 sin Claws of Thunder
270 sin Dragon Tail
271 sin Lightning Sentry
272 sin Inferno Sentry
273 sin Mind Blast
274 sin Blades of Ice
275 sin Dragon Flight
276 sin Death Sentry
277 sin Blade Shield
278 sin Venom
279 sin Shadow Master
280 sin Royal Strike
Note: Holy Shield has itypea1=shld (shield), and that is what is restricting it from spawning on scepters. Other potential staffmods that share a similar problem are Poison Dagger (knife), Smite (shield), Double Swing (melee), Stun (melee), Double Throw (throwing), Leap Attack (melee), Concentrate (melee), Frenzy (melee), Whirlwind (melee), and Berserk (melee).
Also, as previously stated by Adeyke, you can get three skills with up to +3 on each and this process is selected in this manner:
'Staffmod' selection:
First, the game chooses how many skills to add. It will do rnd[100], and add to it the ilvl of the item IF it is being imbued. Then, if higher than 90, 3 skills will be chosen. If higher than 70, 2 skills. If higher tan 30, 1.
After this, it selects the skill id. It will start at a certain value depending on the ilvl:
Above 36: 5
Between 24 and 36: 4
Between 18 and 24: 3
Between 18 and 11: 2
11 or lower: 1
This the becomes a base for all the skill ids. Then, it has a chance to be modified:
20% chance to add 1
50% chance to stay unmodified
20% chance to subtract 1
10% chance to subtract 2
If the item is low quality, then the skill id as of now is capped at 4. Then the final id is chosen from the formula below. skill_adder is the skill id so far, base_id is the starting id for the character class.
id = skill_adder + (base_id + 4 * skill_adder) - 5 + rnd[5]
If this ends as 73 (Poison Dagger), it will do the rnd[5] again. And finally, the total skill bonus is chosen. It will do rnd[100], and add to that ilvl / 2 if from imbue.
If 90 or above, +3
Between 60 and 90: +2
Lower than 60: +1
This applies to:
ItemType StaffMods
Cloak sin <unused>
Hand to Hand 2 sin
Primal Helm bar
Pelt dru
Wand nec
Voodoo Heads nec
Scepter pal
Staff sor
Orb sor
-Hammerman (slightly correct by gta-maloy])
In very rare cases a skill that normally isn't able to spawn somhow manages to appear on an item (a very popular example is holy shield on scepters). It is believe that if the game fails six consecutive checks while selecting a staffmod that the seventh skill will be placed on the item regardless if it is normally possible or not. I would assume that this was implemented to avoid an error that would either crash the game or stop the item from dropping (which in turn might crash the game).
