Eeyore writes:
Why would you need more than one type ?

Most of them are the usual 15 amp outlets (||). Some (in my workshop)
are 15/20 outlets, with the extra slot for 20A-only cords(+|). I have
one 30A twist-lock outlet in my generator, too.

I don't think I've ever seen the 20A-only plug on anything though.
Have they gone the way of the $2 bill?