It was like LGPL in the local case: you alter the library and make it available through the software you link with it, so you have to share the changes you've made to the library, but not the rest of the linked code.
With SSPL, it's like the full GPL in the local case: 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.
Another tricky question is where the border line is. If I write a wrapper that interfaces with MongoDB and repackages its data, then makes them remotely available to the rest of my service, do I only need to share the wrapper? If not, and any network connection that substantially uses an SSPL piece counts, then do I have to share my internal monitoring system? Am I even allowed to connect closed-source data analysis tools to an SSPL database? Etc.