Ok, so, looking at the chart with the armor rules, I see that most of them have a rating for AP at X/Y. For some reason, I can't find a clarification. Is X just the helmet, and Y the rest of the body, or is Y the torso, and X the rest of the extremities?

It is body vs the rest (body with highest).

It is a bit dificult to find, but it is stated under the description of the Astartes power Armour, under "Ceramite Plating"

Thank you very much!