What is uniform and non uniform insulation in transformer terminology? Where do we use non uniform insulation and upto what kV level it is used?

Taken from IEC 60076-3 the definitions are as follows:

uniform insulation of a transformer winding the insulation of a transformer winding when all its ends connected to terminals have the same rated insulation level.

non-uniform insulation of a transformer winding the insulation of a transformer winding when it has a neutral terminal end for direct or indirect connection to earth, and is designed with a lower insulation level than assigned for the line terminal

Uniformly Insulated - A VT having all primary connections of equal or rated insulation level. Non-Uniformly Insulated - A VT having one primary terminal intended for direct or indirect connection to earth, which terminal being assigned a lower rated insulation level than the other primary terminal(s).