example for another skill: Crucify
Shadowsabre
Shadowsabre
Crystal Sword (Sacred)
Required Level: 100
Required Dexterity: 510
Item Level: 105
Innate Cold Damage: (45.0% of Dexterity)
Crucify: +14% Magic Damage per Base Level
50% Hit Recovery
Adds 125-250 Fire Damage
Adds 125-250 Lightning Damage
Adds 160-250 Cold Damage
-20% to Enemy Elemental Resistances
+(15 to 25) to Crucify (Assassin Only)
Fire Absorb 5%
Lightning Absorb 5%
Cold Absorb 5%
+4 Life on Striking
Socketed (6)
vs
The Captain
The Captain
Military Pick (Sacred)
One-Hand Damage: (136 - 188) to (234 - 290)
Required Level: 100
Required Strength: 500
Item Level: 105
Strength Damage Bonus: (0.11 per Strength)%
(10 to 40)% Attack Speed
Adds (40 to 70)-(120 to 150) Damage
+(120 to 170)% Enhanced Damage
+(14 to 20) to Crucify
+2000 Defense vs. Missile
+100 to Vitality
Socketed (6)
You can see the difference now with those items. One grants a skill directly for assassin, whereas the other grants it for all classes to use. When oskills are used on the class that already has that skill, it caps at +3 unless it directly says +xx (class only)