"Basically, when and why wee need to more restiriction than master envelope?"
When there are uncertainties in the method you use for calculating your CoG.
E g if your method of calculating the index generated by the pax distribution can possibly give you an index which is x units higher than the true index, you need to reduce the index limit in the envelope used by x units. This curtailment applied for pax, cargo, in flight movements and what do you have will change the envelope as seen "on the sharp end" - even though the master envelope remains the same.
The point is to use an artificial envelope which has enough margin to the actual envelope to to make sure that the possible errors in the calculations used can never put you outside the actual envelope.