Question:

How is calib from the wfdisc table used?

Answer:

The value in the wfdisc table is used by dbpick and other Antelope programs.

There is no value of calib in a miniseed volume.

The corresponding values (not calib) which are put into a seed volume by db2sd come from the stage table, though there's a sanity check that the value from the calibration table is equal to the (calculated) value from the stage table.