Obviously, you've never been to Signapore!
J/k
A resupply ship stationed at various ports or trans-Atlantic locations, designed to allow the subs to extend their patrols.
The game though, treats these as ports, and SHC will reduce the number of days you spend at them, but you are forced to start a new patrol.
So if you went out on patrol, and the base was changed, yet you returned to the original port, it would be treated as a 'foreign port'. So travel to the new port, and see what you can kill on the way.
Were you able to to change anything on your boat at base before you sailed?