Virtual parts in videos are divisions throughout a video that can only be seen by the channel team. By having these parts, this allows for multiple users to work on the same video simultaneously.
Virtual parts can be accessed via the Subtitle Editor or Segment Timer, using the navigation in the lower left-hand corner of the volunteer tools.
The subtitling progress of each part can be found in the upper right-hand corner, when a part is selected. (Tip: When subtitling, it's courtesy to let people know which part you're working on as well as the progress you've made through the "Team Discussion." For example, you can say you've worked on Episode 1, Part 3 and have completed up to 86% of the part.)