Depending on its mode, the smart:filter either only tries to fix imbalances of the current track or it also takes potential masking problems with other tracks into account (when part of a group).
In track & group (tr+gr) mode, the smart:filter looks at spectral imbalances of the current track as well as potential masking problems with other group members. This is the default mode for smart:EQ 4.
In track mode, the smart:filter only looks at spectral imbalances and problems of the current track. Potential masking problems with other group members are ignored. Even when the plug-in is a part of a group, you may want to use this mode if you do not want other tracks to have an impact on the sound of a certain track.
In group mode, the smart:filter only checks potential masking issues with other group members, but does not try to fix potential spectral imbalances of the track itself. Therefore, this mode only shows a filter curve if the plug-in is part of a group. You may want to use this mode if you want to stick to the overall sound of track and only want to embed it a bit smoother into a group of multiple tracks.