Could be in two ways
1) SDM Event (upon some event, like a ticket Close) to trigger a PAM process.
2) PAM process uses the SDM Ticket #, queries for the Catalog request id associated with it (OOTB, its in Call_Req.external_system_ticket)
3) Makes a Catalog SOAP call to close the Catalog Request
OR, this can be done on a frequent basis (every 3 hours or once a day) where you can query a table to identify what SDM tickets were closed that day, and check/close associated requests in Catalog.
_R