Weapons with the "local ssm" are just like regular weapons, fired on the field at a target. I think their behaviour is a bit bug-y if it is a swarm missile.
SSMs defined in the table are fired in salvo outside the mission area on a TAG-Ced ship.
In order to do that, you need :
-a regular missile defined in your weapon table
-define how that missile is fired in the ssm table (note that you can have multiple entries for a same missile, with different strike behaviour)
-and a level 3 TAG weapon, calling your ssm strike by index number from your ssm table.
oh, and I'm not 100% sure, but I think you can't fire more than 10 missiles per salvo