Variable file names

Please help.

I am creating (attempting to, anyway) a sub-procedure that receives a
file name as a parameter.
This file will be chained to in the sub-procedure.
The file names are not known until the calling program is being run, as
they are based on a date in records processed by the calling program.
All the file names that are passed refer to files that are 'archives' of
the same externally DDS described file (PPXC00).

The manual says:
1....2....3....
4. This keyword is not used to find an externally-described file at
compile time. 
What exactly does that (#4) mean anyway?
If it means what I inferred (that it is NOT going to go look for a
definition) why does it blow up?
(The archive example I saw was just a code snippet, but it had the file
externally described.)

Is there a way to code this and have these files be externally
described?
************************************************************************
***********
This does not compile:
 FPPXC00    IF   E           K Disk    ExtFile(FileName)

 F                                                 UsrOpn     

/Free                               
/Copy EXPRDPGM/QRPGSRC,PROTSP           (contains PR for sub-procedure)

/End-Free                                                        (field
'FileName' defined here also   )
                        
D AdjCst          PI            10S 4                    
D Prt#                               Like(PtPrt#)       
D CYMD                          8S 0                    
D FileName                     20A                      
 
Some calcs here....
                                                      
Compile error:
*RNF7030 FILENAME          **UNDEF**               9           ("9" is
the PPXC00 F-spec)                             

TIA

Tim Kredlo 
Ps: I did look in the archives. Did not see and example of the name
being passed in