Files are referenced by numbers in PCC. You normally use the FreeFile() function to obtain a currently-unused file number. (Alternatively, you can hard-code file numbers, but this is not recommended.)
By convention, file numbers are written with a leading hash sign:
Dim fd = FreeFile() Open "test.txt" For Input As #fd
In most cases, however, it is clear from context that you mean a file number, so you can omit the hash sign. The only case where the hash sign is required is the Print function.
Version Differences: The number of allowed files differs between PCC versions. PCC2 allows file numbers from 1 to 100, PCC 1.x only allows 1..20. (If you use FreeFile to obtain file numbers, you will not notice this difference, except for being able to open more files.)