As for how they protect, if something can't penetrate the armor it won't cut you, however kinetic energy always travels through armoring as well but is dispersed. I'll try to write up some more details on the system.
As for armor parts and what they protect here's a list I have written down a while ago.
Armor parts:
Head:
-Top shielding
-Back head shielding
-Visor
-Nose protector
-Mouth protector
-Side shield
-Neck shielding
Body:
-Chest plate L,R
-Stomach shielding
-Back shielding
-Lower back shielding
-Crotch protector
-Hip protector
-Neck shielding
-Deflector (chest)
-Shoulder protector
Shoulders:
-Shoulder cover
-Neck protector
-Upper arm protector
Arms:
-Upper arm plating outside/inside
-Elbow protector
-Lower arm plating outside/inside
-Wrist shield
-Hand shield
Gloves:
-Back hand shielding
-Palm armoring
-Finger shielding
-Finger armoring
-Wrist armoring
Hips/crotch:
-Lower body shielding front/back/sides
Legs:
-Upper leg armoring outside/inside
-Knee protector
-Lower leg armoring outside/inside
Feet:
-Foot shielding upper foot/sole
-Ankle shielding
Connected parts:
Face plate; Nose protector – Mouth protector – Visor
Solid cuirass; Chest plate L, R – Stomach shielding – Upper + Lower back shielding
Two part cuirass; Chest plate L, R – Stomach shielding; Upper + Lower back shielding
Head helmet; Top head shielding – Back head shielding
Full helmet; Top head shielding – Back head shielding – Side shielding
Closed helmet; Full helmet – Face plate
Examples:
Legionary Helmet:
-Parts; Top head shielding – Back head shielding, Side shield
Hoplite Helmet:
-Parts; Top head shielding – Back head shielding – Side shielding – Nose protector
Corinthian helmet:
-Parts; Top head shielding – Back head shielding – Side shielding – Face plate