If all of the processing that results from a ca_put_callback() is traced by EPICS, the callback from EPICS is sufficient to indicate completion, and use of the BUSY record is not warranted. But if you want to include some processing that isn't traced by EPICS, the BUSY record is a convenient way to fold it in. If you arrange for a BUSY record to be processed as a traceable result of the ca_put_callback(), and cause its VAL field to have the value "Busy" (1), the BUSY record will pretend to be doing something (i.e., it will not execute its forward link) as long as its VAL field remains "Busy". When VAL is set to "Done", and the record is processed, it will execute its forward link, notifying EPICS that it is finished, and EPICS will send the callback.
The write that sets a BUSY record to 1 must be done by either a ca_put_callback(), or a PP link. (Note that several records in the synApps package of EPICS-application software have output links that are capable of issuing a ca_put_callback(), and that some of them do so via a CA link. Don't be fooled by this. The trick is performed by device support, which calls dbCaPutLinkCallback(), rather than dbPutLink(). dbCaPutLinkCallback() queues a request that eventually results in a ca_put_callback().)
The write that resets the record to 0 can be done by a channel-access client, or by device support. If it's done by a CA client, it must be done by either a ca_put() or a CA link.
The BUSY record is essentially a copy of the BO record from EPICS 18.104.22.168, with two modifications: (1)the process routine declines to call recGblFwdLink() when the VAL field has the value 1 ("Busy"); (2) the ZNAM and ONAM strings have the default values "Done", and "Busy", respectively.
For version 1.2 of the BUSY module, Ben Franksen added "Raw Soft Channel" device support, which allows the BUSY record to write a user configurable integer value (the value of the MASK field) via its OUT link, when the record's VAL field has the value 1. Currently, MASK can only be specified at DCT time. For example, you can configure MASK to a bit-pattern and have the BUSY record write this value to a selRecord's SELN field.