The None domain is not very useful for interactive work but it is provided nevertheless for completeness of the FriCAS type system.
Probably the only place you will ever see it is if you enter an empty list with no type information.
Type: List None
Such an empty list can be converted into an empty list of any other type.
[ ] :: List Float
Type: List Float
If you wish to produce an empty list of a particular type directly, such as List NonNegativeInteger, do it this way.
Type: List NonNegativeInteger