SONAR has always considered sidechains from the point of view of what you want to route to the sidechain - i.e. the sending track or bus. Users of some other steinberg DAW's are used to seeing sidechain from the point of view of the plugin and picking the source from there. Its just a matter of semantics.
Personally I find it more natural from a routing point of view to think of it from what you are sending to the sidechain. However since some users prefer to look for it at the plugin level, we may consider adding a sidechain picker to the UI in the future.