In article ,
says...


I think it serves when you have two airspaces with no margin between them.



If you see, say, 50/SFC for a Class C, and 80/50+ for a Class B above it, it
means that the Class C extends from the surface to 5000 feet inclusive, and
the Class B extends from 5001 feet to 8000 feet inclusive.

Without a plus or minus sign, there is an ambiguous margin of 100 feet between
the airspaces. For example 50/SFC for the Class C and 80/51 for the Class B
means that the area between 5001 feet and 5099 feet inclusive is in neither
airspace. Since this could cause problems if someone were to actually try to
fly through this thin slice of air, calling it uncontrolled, the + and - are
used to make it clear that the two airspaces touch each other, with no space
between.


There is never any space "between" overlying airspaces.
There is no "ambiguous margin". Airspace altitudes are charted and unambiguous.
If Class B is charted at XX for a given zone, and you fly higher without being
cleared into it, do not expect the FAA to look at you with pitiful indulgence
while you roll out your "ambiguous zone" theory.

In the case you cite, where one ceiling is lower than an overlying floor, the
space between is also readable on the chart - usually Class E, which is not
uncontrolled airspace.

Class D underlying Class B has its own ceiling, also clearly charted.

Pilots know how to read the charts. Fortunate we are that you do not fly
airplanes, and we do not have to worry about sharing the airspace with you and
your fuzzy theories.