"Merge and lock markers" vs. "Apply transient pool markers"

Off the top of my head, there are three main differences:

1. Merge and Lock will take active transient markers on all selected tracks, and copy them as User markers to all other tracks, whereas Apply Pool only goes one direction (from the pool to the selected target track). To get the same effect for 5 tracks, for example, you'd have to use Apply Pool five times, selecting the complementary four tracks  to add to the pool each time.

2. The "Lock" part of Merge and Lock actually refers to locking the position of the audio clips, which could be done in conjunction with Apply Pool, but would be  separate operation.

3.  The Applied markers are not linked in the way that merged markers are, so moving or stretching a marker in one track will not be reflected in the other track(s).