Compatible Characteristics

For format-F records, if a data set has an LRECL value that differs from the value in the DCB, the block size for that data set is not considered during OPEN.

For format-V records, if a data set has an LRECL value that is larger than the value in the DCB, the block size for that data set is not considered during OPEN.

A RECFM value of U in the DCB is consistent with any other RECFM value.

BSAM considers the following RECFM values compatible with the specified record format for the first data set:

F or FB—Compatible record formats are F, FB, FS, and FBS.
V or VB—Compatible record formats are V and VB.
U—All other record formats are compatible.

If OPEN finds an inconsistent RECFM, it will issue a warning message. OPEN does not examine DSORG when testing consistency. It does not issue ABEND since you might not read as far as that data set or you might later turn on the DCB unlike attributes bit.

Even though RECFMs of concatenated data sets can be considered compatible by BSAM (and you do not receive the expected warning message) that does not guarantee they can be successfully processed. It still can be necessary to treat them as unlike.