All melee attacks are governed by the attack table. Each value in the table is absolute, a 10% chance to block means that all attacks have a 10% chance to be blocked, not just ones that hit, aren't dodged, etc. The server rolls a value between 1.00 and 100.00 and will select the corrosponding attack on the table.
Each point in defense increases your chance to be missed, dodge, block, or parry an enemies attack by
Each point also decreases the chance you are crit by
This makes every ~8 points of defense ~1% avoidance.
A level 63 mob has a
5.60% chance to crit you so at 440 (+140) defense their chance to crit you
High enough defense or spells such as
Shield Block can "push" lower priority attack types off