In Animation Desk, there are two ways to tag frames, put labels of different colors on the frames, and add descriptions.
Use Timeline to Tag Frames
You can select a single frame on any timeline, click the "Tag" icon, and select the desired color label.
Use Frame Overview
1. Enter the frame overview (select the settings button in the bottom right).
2. Check the box in the top right and select any frame you want to tag.
3. After selecting, you can click the icon in the upper right corner to tag.