FAR 25 defines a set of loads (not just g loads) that (nominally) may be expected once in the life of an airplane. These are limit loads. The structure must be demonstrated by test to be able to withstand 1.5 times these loads.
These limit loads are defined against structural design speed requirements Va,
Vb,Vc and Vd - there are no such speeds as Vno and Vne in FAR 25.
Va,b,c,d speeds are defined in 25.335
The requirements associated with Vd/Md have to be demonstrated by flight test if Md is greater than 0.8M. FAR 25.629 defines an enlarged flight envelope beyond Vd/Md within which certain aeroelastic criteria must be met.
Obviously any margins required over the flight demonstrated limits have to be "proved" by calculation. The aircraft must shown to by calculation be free of flutter, control reversal etc. up to 1.15 Vd/Md (unless this speed would exceed Mach 1.0)
I see this 1.15 factor (was 1.2) as similar in concept to the 1.5 load factor of safety but of course it is a 'paper' margin where the 1.5 factor has to be physically demonstrated.