Add-in functions have two names: the native 'C' name for the function and the name under which it is exported to Excel. Usually the two names are the same, but they do not need to be.
The 'C' name should be legal 'C' name, starting with an underscore or an alphabetic character and containing only underscores and alphanumeric characters.
It should be unique within any single source file, or it will be rejected by the XLL+ Function Wizard. (Of course it should also be unique within a project, but this is an issue dealt with by the linker.)
By default the Excel name is the same as the 'C' name. You may wish or need to change the Excel name to avoid conflicts with Excel built-in functions. By convention (but not by necessity) Excel worksheet functions are in upper-case, and you may wish to follow that convention.
The Excel name can be changed in the XLL+ Function Wizard.
Excel names should contain either 'C' names or a sequence of 'C' names separated by periods, e.g. AFunctionName, _Func1, Function.Name, GET.CURRENT.VALUE.
Excel names must not conflict with cell addresses, e.g. A1, ZZ345.