Melee Attack determines the chance a unit has of hitting an enemy in melee.
All units have a base 40% chance to hit an enemy, with every additional point of Melee Attack above the opponents Melee Defence score increasing the chance by 1%. The minmum hit chance is 10%, and the maximum is 90%.
40 + (32-6) = 66%
The Zombie attacks back. It has 5 Melee Attack versus the Temple Guard's 38 Melee Defence. It has a chance to hit of 10%.
40 + (5-38) = 7% This is below the 10% minimum so gets rounded up.
Bonus vs Infantry/Large
Bonuses vs Infantry or Large models provide an additive bonus to Melee Attack and Damage.
The Temple Guard attacks a Reiksguard Knight. The Temple Guard has 32 MA and a +16 bonus vs Large. The Reiksguard Knight has 28 Melee Defence. The Temple Guard has a 60% chance to hit the Knight.
40 + ((32+16)-28) = 60%