These are accounting files which ODEX spools off periodically. They contain information relating to connections, file transfers etc.
Some ODEX users process these to obtain statistics or to generate billing information. If you have no interest in doing this you can just delete them and change the ACCOUNTING flag in ODEXCC.PRM to N.
The format of the records in the ACC file is defined in the C language header file acct.h (which should be in the ODEX installation directory).
The most recent file which ODEX is currently using is called ODEXCC.ACC. You can delete this when ODEX is stopped (the flag change only takes effect when ODEX is restarted).