Here’s what they have to say about the new feature:
As of Version 3, Link allows peers to share information on the user’s intent to start or stop transport with other peers that have the feature enabled.
Start/stop state changes only follow user actions. This means applications will not adapt to, or automatically change the start/stop state of a Link session when they are joining. After a peer joins a session it exposes and listens to all upcoming start/stop state changes. This is different to tempo, beat, and phase that are automatically aligned as soon as an application joins a session.
As every application handles start and stop commands according to its capabilities and quantization, it is not expected that applications start or stop at the same time. Rather every application should start according to its quantum and phase.
Ableton Link 3.0 is backwards compatible with earlier versions and is already available in Audiobus 3, and Audiobus compatible apps get Start Stop Sync support ‘for free’.
Developers can get details via github.