They aren't variables, they are strings.
I don't know much about the program you've mentioned, but the code snippet is building a string of text called strA from the other strings and adding a few characters to format it. If you were calling me, I think:
strCallsign = "PU5TOK"
strTo = "KK4AMP"
StrDigis = "Hello there!" (I think. someone else check me here if they know more about this particular program)
strA would be created as blank. The first line after the DIM for it would make strA become:
If strDigis was empty, strA would be unmodified by the next line. However, it isn't in this example so strA becomes:
PU5TOK>KK4AMP, Hello there!
strA then gets a colon ":" and telemetry added to the end of the line. Don't ask me about telemetry, 'cuz I dunno yet.
The last line of the subroutine appears to print the entire theng, then a CR/LF to be ready for the next print on a new line.
[SIZE=1][B]"An armed society is a polite society. Manners are good when one may have to back up his acts with his life." - Robert A Heinlein[/B][/SIZE]