A project in Video Slave basically contains two things: media and timelines. When you add media to your project - either by dragging and dropping it to the media section of the main window or by choosing Add Media to Project from the File menu - it is stored in the media bin and will be restored when you reopen your project.
The other thing are timelines. A project in Video Slave can contain a number of timelines. Each timeline in turn can hold multiple video and audio tracks. Depending on your personal preference and the kind of project you're working on, one or several timelines per project may be the best choice. There's no right or wrong about the number of timelines used per project.
When you create visual events, they live on the event track of the currently active timeline. If you switch the timeline, the events will switch as well. Hence, timelines allow for a clear separation of visual events. Again, depending on the type of project you're working on, this may be just what you want or not ideal. If you want don't want your cues separate for e.g. each reel of a multi-reel feature film, you can simply stick to one timeline with all reels on one video track at the respective hours.
All Video Slave versions up to version 3 used a playlist-based approach. You just pulled your movies into the application, assigned a start timecode and Video Slave switched to the correct reel according to the incoming timecode. This was just what many of our users wanted and it was a great approach except for one thing: editing.
Editing video, e.g. inserting a 30 second clip with a newly rendered scene into a reel was not easily possible and it was becoming a growing problems for our users. So we decided to switch to a timeline-based approach with version 4. To give our users the most flexibility, we wanted to offer the possibility to create several timelines per project. Using multiple timelines is however not required in many scenarios.
Here's a recommendation of what we think makes the most sense depending on the type of project you have:
If you work on feature films with multiple reels, documentaries or similar with only one movie asset, one timeline will probably be the best choice for you. If you're working on a feature film with reels starting at the respective hours, just make the timeline long enough (switch to the timeline tab in the top left of the main window and click the "Edit" button on the timeline) and then place the movies at the respective hours. Video Slave will then behave the same way it did in earlier versions and switch to the corresponding movie following the incoming timecode.
If you ever get inserts with e.g. a rendered scene for a reel, just create a new video track and drop the insert on there. The same applies if you get a new picture cut. You can either remove the old one from your main video track or - alternatively - just create a new video track, move the new cut onto that and then disable the old video track.
If you work on e.g. a TV show where all episodes start at a timecode of 10:00:00:00 for example, it can get a bit fiddly in one timeline as you would need to create a separate video track for each of the episodes. This works of course but involves a lot of scrolling and muting/soloing video tracks. Using one timeline per episode might be cleaner for that use case.
Another use case for adding a second timeline is if you're working on a reel based feature film but want to create a tied version of the same movie files to e.g. check music at the reel boundaries. In that case you can just create a second timeline, drag the same movie files onto the first video track and tie them together by removing the leader and black (if any) after the last frame of action and arranging the regions next to each other. You can then either drag the bounced music directly into Video Slave or build a session in your DAW - whatever you prefer.
These are of course only recommendations for the most common use cases. Please feel free to use timelines how they work best for your project!