writes:

What is the practical difference between "above, but not
including" (e.g., +12) and "above and including" (e.g., 12)?

I think it serves when you have two airspaces with no margin between them.
For exampple, one is 12/SFC, the other is 70/12+. So at 1200 feet you're in
one airspace, and at 1201 feet, you're in the other. If they were specified
as 11/SFC and 70/12, the space between 1101 feet and 1199 feet inclusive would
be outside either airspace.