S

is the type of the accumulator (and therefore also the return type).

Yeah T is the type passed as the Type in the List but we aren't passing S anywhere

With

reduce

the accumulator type is determined by the first element of the list being processed. I’ve linked an example: https://pl.kotl.in/HyrqmCweN

You're passing

S

as the type of the first argument in the lambda. This can be any supertype of the collections element type: https://pl.kotl.in/rk0XOTwx4

Thanks a lot:)