If you do not specify an interface for a printer when you create it with lpadmin then /usr/lib/lp/model/standard will be selected. This file is copied to /etc/lp/interfaces/<printername>. If you do specify an interface then it is copied to /etc/lp/interfaces/<printername> (??).
lpsched passes the print job to /etc/lp/interfaces/<printername>. lpsched sets $FILTER to indicate which filter or combination of filters is needed to process the print job.
Two filters makes $FILTER look like "filter1|filter2". That's a pipe in there.
If two single filters will work, lpsched takes the first one in the filter table.
If a single filter or a combination of two other filters will work, lpsched takes the single filter.