I don't generally agree with ZeConster on this. Since 'Custom Protection' is a distinct Feature of certain armors, the best thing to do is look at the examples given - and why each example is that way. Note that not all the examples are of a single type; the Evo HEL Suit, for example, possesses Custom Protection (Insulation 4, Fire Resistance 2, Radiation Shield 3, Chemical Protection 2) - four different types. However, the HEL suit is a space-suit, and kind of requires all those different kinds of protection. The Arctic Diving Suit has Custom Protection (Insulation 8, in water only) - which makes sense for a diving suit meant to be used, y'know, in the arctic.
The issue that seems to be vague is the Globetrotter / Survivalist / Big Game Hunter versions, which are marketed specifically for people to get their own 2- to 6-point-resistance version. Those seem to be intended to match up to a particular environment - insulation for a cold enviroment, etc. Considering the other examples of Custom Protection, though, I personally wouldn't require those to be 'all one thing' - because hey, you may be wanting to go into a toxic jungle, and thus need both Fire Resistance and Chemical Protection.
It DOES restrict the options there, though - so I probably wouldn't permit Nonconductivity, only Fire Resistance, Insulation, Chemical Protection, and maybe Radiation Shield.