Since this rule is being introduced new, you have what you have. Breastplates are counted as Full Plate Body with no arms or legs, and get armor rating tied to having just one item - they could add plate arms and legs and upgrade into full plate, or add in some chain and become half-plate.
Since Gillen is wearing Half Plate, he's wearing Plate Arm and Plate Body, along with Chain Leg. So he could upgrade to Full Plate just by exchanging the legs.
In the case of Bardings, then "arm" is considered the skirts that cover the front legs, and "leg" are the skirts covering the rear legs, with the skirts extending up far enough to cover parts of the flanks and whatnot.
Called Shots on Piecemeal armor would work the same. If you're wearing plate arms and no legs, and get a called shot to the leg but it misses, then it's assumed you got in the way of the blow and put your shoulder into the blade, deflecting it off the heavy armor before the swing went low enough to hit legs.
NINJA: Called Shot penalties vary by difficulty - if you've played DF Adventure Mode then you understand. Hitting an arm is -2 attack, hitting an ear is -10. Ranged and Reach attacks get -2 on top, and Ranged attacks get double range increment penalty and an additional -2 for being from a distance. So striking someone's arm with a sword is -2, striking their arm with a lance is -4, striking their arm with a longbow at 50 foot is -6, and striking their arm with a longbow from 150 feet would be -10. I think. Effects like True Strike are only used to make traditional "overall body shots" - but the DM can decide if these 'automatic hit' abilities are able to make called shots or not. Depends if we want people taking arrows to the eye when an archer has True Strike.