aPC and gPC

Hi uqlab!

I have some question about arbitrary polynomial chaos expansion.

Unlike gPC, aPC knows that unlike gPC, it does not assume input distribution, and uses only data to create a PCE model with the basis as the stielgjes procudure. So, in my opinion, we only need to declare the lower bound and upper bound of the variable. In UQLAB, we must specify the input distribution as shown in (a) in the figure. Is this just a process for constructing a model in uqlab? For example, what is the physically difference between (b) (with a uniform distribution) and (a) ?