I was going to post this as a response to a question in the Record->Overdub->Play thread, but it's going to go on pretty long, and stands on it's own as an introduction to record synchronization for newer users. It's really a draft of what should be in the documentation that doesn't exist yet.

"What's the difference between Track sync and Master Sync? and what's the difference between sync unit and track sync unit?"

I'm going to reply to this in several parts since it's a topic that can go off in a lot of directions, so I'll try to each part short and easier to digest.

Let's start with Track Sync. This is the oldest form of synchronization, and what I think most people have used over the years. You start by recording one track freely at any tempo by pressing Record twice. Recording starts as soon as you press the Record button and ends immediately after pressing Record a second time. This creates the "master track". You can now record other tracks that will play along in sync with the master track. They will remain in sync because they are forced to have a length that is the same as the master track, or some multiple of it, like twice as long, three times as long, etc.

Let's call these other tracks "follower" tracks. When you send commands to a follower track, rather than those command happening immediately, they can be delayed until the master track plays to a certain location. The most common location to wait for is the "loop point", that point in time when the master track has played all the way to the end and loops back around to the beginning.

When you use the Record function in a follower track, the Record doesn't happen immediately, it will wait for the master track to reach the loop point. Similarly when you end the recording in a follower track, the recording continues until the master track reaches the loop point. This results in a new track that has exactly the same size as the master track and they can play together without drifting apart.

To accomplish this you need to configure two parameters in each track. This configuration is done in the Edit Session window. First pick the track you want to be the master track. This is usually track number 1 but it could be any track. Select that track in the track table, and click on The "Sync" category in the tree, and a form is displayed containing a field labeled Sync Source. The default value for Sync Source is None which is what you want for the master track. You don't want it to synchronize with anything, you want it to be the leader for the other tracks. If you have changed this to something other than None in the past, set it back to None.

When you look at forms in the session editor, you will notice that some of the parameter names have grey text, and some have a brighter orange color. If a parameter has a grey name it means it is using the default value and cannot be changed. If the default value is what you want, then leave it that way. If you want to change it, click on the parameter name and it will turn orange, then you can change the value.

Now, you need to make change to all of the tracks you want to follow the master track. For most people, this means all of them so you will need to repeat this process in every other track. Select the follower track in the track table, and click on the Sync category in the tree. Go to the Sync Source field and click on the name so that it turns Orange. Set this value to Track. What this says is that you want this track to synchronize with the master track. Or what we casually call on the forum "track sync".

Next, find the parameter named Track Sync Unit. This is where you pick the location you want the master track to be at when you start or end the recording. If you want to start/stop at the master track's loop point, select Loop. You can also pick smaller units Cycle and Subcycle but those are less common. This gets to your question about the difference between Track Sync Unit and Sync Unit. Track Sync Unit is only used when Sync Source is set to Track. If you set Sync Source to anything else, then Sync Unit is used instead. I'll explain why later, but for now just remember that Track Sync Unit goes with Sync Source=Track.

Once you have the follower tracks configured with SyncSource=Track and TrackSyncUnit=Loop, click Save in the session editor. Now record the master track. When you record the follower tracks, if the master is not at the loop point you see the word Synchronize in the center of the UI. This means it is waiting for sync source to reach a location. As soon as the master track reaches the loop point, Synchronize changes to Record and recording begins.

If you keep the recording going beyond the master track loop point, the follower track will start accumulating cycles. You can record for as long as you want, and every time the master track plays to the end another cycle is added to the follower. Each cycle will be exactly as long as the master track. When you've recorded enough, press the Record button again, and the follower again enters Synchronize mode waiting for the master track to reach the loop point. This is sometimes called the "round off period". It sures that the follower track will be an exact multiple of the length of the master track.