A basename must neither contain spaces nor any special characters such as ":/()#?*...".
When your program is a single task application
(i.e. Application.SingleTask is
base name will be used without further modification.
Otherwise, it gets a ".1", ".2", etc. appended, depending on how many applications are already running. If you need to know the name of your ARexx port, you can query the base name attribute after the application is created.