From Famitracker Wiki
This page regroups global effects. These are effects that affect the whole track, and can be used on any channel's effect column.
Bxx jumps to frame xx. Its main use is to make tracks loop at a frame other than the first one.
Cxx effectively ends your track at the line it's used on. xx has no effect whatsoever. Use this if you want to prevent your track from looping endlessly.
Dxx causes the tracker to skip to the next frame and start at row xx. Bearing in mind that the amount of "Rows" under Song settings is global, this can be useful for making a specific pattern shorter than the rest.
It should be noted that there is a bug that prevents skipping to a row that is further down in the next pattern than the line it's used on. Basically, xx cannot be larger than yy (the row number it is used on), or else xx will be ignored and the tracker will skip to row yy instead.