Patchwork Armor (described on page 189 of the sixth printing of Tactical Operations: Advanced Units & Equipment) allows some locations of a unit to have different armor on than others. Based on both the text and posts by writers in this forum, this is mainly intended to allow repairs using salvage when supplies are slim. Indeed, as a Class A refit (page 211 of the fourth printing of Campaign Operations), this is generally quite achievable.
The example provided has a Griffin 1N with standard armor mounting salvaged ferro-fibrous armor on its right arm. But the reverse is equally plausible, and this produces an ambiguity. Consider a Griffin 2N with ferro-fibrous armor: logically, a technician in a supply crunch (perhaps during the early Succession Wars) should be able to mount salvaged standard armor (which occupies no slots) on the right arm as well. To avoid being overweight, of course, this necessitates replacing 18 points of ferro-fibrous armor with 16 points of standard.
However, the resulting design has no room for the original ferro-fibrous armor's critical slots under Patchwork Armor rules on the center torso, head, or right torso. They only fit using the non-Patchwork critical allocation that the Griffin 2N was originally constructed with. Does that mean it is prohibited?
For the sake of both logic and allowing for field refits like that, I believe that the use of Patchwork Armor in some locations should not be prevented by the existing "base" armor used on other locations. Instead, only altered armor types should require more critical slots. This is even more inefficient than the alternate interpretation of using patchwork rules for every location, mind: you have to allocate the full critical slots of the base armor type, and the additional slots (if any) of each type applied as patchwork.
While not strictly relevant, it is worth noting that this is the interpretation MegaMekLab currently uses.
