Guides are labels on the timeline that can be added by right clicking at a spot on the timeline scale and choosing . You can put a comment in the guide and make the comment display by choosing Show Marker Comments in the menu or by clicking on the Show Marker Comments button.
Guides in the pic below are the purple flags. Not to be confused with markers (gold in the picture below). Guides are static on the timeline and are stationary when clips are moved around. Markers are inside the clips and move with the clips.
Guides can be used to define regions for rendering - see Rendering With The Guide Zone Option
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