> if you take the licensed software, and link it (via rpc / network) to your other software, you must share not only any changes you've made to the licensed part, but the whole thing that uses it.

Slight modification makes this accurate to the SSPL: if you take the licensed software, and link it (via rpc / network) to your other software which you use to offer the SSPL licensed software as a service, you must share...

> Another tricky question is where the border line is.

Ultimately the trigger of the SSPL is whether what you offer publicly is the SSPL-licensed software as a service. It doesn't trigger the SSPL if you make MongoDB available as a service to your application internals as long as what you're making available publicly is not "a service the value of which entirely or primarily derives from the value of the Program or modified version..."