I think it should do what it's name implies and just literally export ONLY what is selected to avoid confusion
As for sequences, maybe those could be exported despite whether or not the markers are selected in the timeline, just as long as the object and it's node that are a part of the sequence is selected, and if there's any objects/nodes that are also a part of the sequence, but are not selected, exclude those
Or it could be a separate option to include only selected or all nodes upon export, having all nodes export could be useful for player animations, but having only selected nodes export could be useful for weapon animations
Like say the player imports the shiftAway animation to the player model and they animate their weapon to go along with the shiftAway animation, that's a scenario where including only selected nodes would be useful