Hail!
I know that we have refit / custom rules for 'Mechs (p188 and follows.) I'm sorry to have to ask such a broad-scope question, but:
How does the system change for units that don't use 'Mech critical-slot rules, such as combat vehicles and aerofighters?
In particular, the definitions of refit classes A, B, and C seem to depend on 'Mech critical slots, with A and B covering most swaps that don't increase used critical space, and C covering most swaps that do.