Those numbers represents the number of 1/10'000'000 of second passed from the start of the movie.
Every frame (in a movie encoded at 25fps) is shown every 40ms, so if you'll take a bookmark after 1frame the saved number will be 400000 (400000/10000000=0.04 that represents 40ms)
I think
BST (BSplayer creator) decided to use 1/10'000'000 values (instead for example of using ms) due to the number of digits resulting from 1/23.976fps = 0.0417084, which if multiplied by 10'000'000 results in 417084 (no decimal numbers). In fact if you save a bookmark after 1 frame in a movie coded at 23.976fps the number saved will be 417083 (~417084)
