Basicly it's all abot how different the two sides (L/R) get compressed.
When linked 100 % both sides get compressed evenly when a signal in one side, either left or right, exeeds the threshold. In IND mode it's the opposite. So if you have loud sound in the left side and a quiet in the right, only the left gets compressed.
The filters are just like a normal side chain and filters out for example the low end in the sidechain, making the compressor react less to low end.
Soundwise I find that a link of about 70% can material a bit wider while still having the center locked. A link of 100 % keeps the center dead tight.
Each setting has it's own uses. But the good thing with the API2500 is that everything sounds amazing
As I understand it it's no more than that?! It means you can have for example a Beatles panning thing going on with loud kick in the left side andn not have it trigger the link, allowing for even more fine tuning than ust the link.
Or inversely you can prevent a hi pitched snare on the right trigger compression on the right side.
That's how I understand it. I gotta admit I haven't used the link filter yet though. But plenty of less than 100 % link for more open material.