The Play Loop button has a specialized function in Color Correction mode. The Play Loop button does not appear in the Composer window but does control the playback of material in the Composer window. You can access the Play Loop button from the Play tab of the Command palette or from the keyboard if it has been mapped to a keyboard location.

When you click the Play Loop button, the system plays the whole sequence in the active monitor, starting from the current position of the position indicator. Playback is not limited to the current segment alone, regardless of the Source menu command selected for the monitor. This is useful whenever you want to view the whole sequence quickly without switching monitors or making a new Source menu choice.