From FFXI Wiki
Job Trait Information
Description Grants an accuracy bonus to melee and ranged attacks from behind an enemy.
Type Merit
Job Thief
Merit Points
Group 2 Ranks Available 5
Effect of each Rank Adds 3 Accuracy and Ranged Accuracy per merit level when behind the monster.

Equipment Modifying Ability...
Equipment Piece Modifier
Assassin's Vest +2 Augment-Icon.png: Gives +1% Triple Attack per merit level when Ambush conditions are met.