Audiobus 3 is an app that uses IAA to host apps provided the IAA app developer included Audiobus support into their app for it to run inside Audiobus (using the free Audiobus supplied SDK).
Audiobus 3 hosted apps can sync to the Audiobus tempo provide the IAA developer added that functionality via the Audiobus SDK.
Link can be enabled and disabled in settings.
Audiobus State Saving means the IAA being hosted will have its settings saved when you save a preset for a setup in Audiobus 3. For example, a synth app would have the preset you were using saved when you save the setup as a preset in Audiobus.
Not all apps with Audiobus support have state savings.
Some apps can be both IAA hosts and have Audiobus support so they can be hosted inside Audiobus. Example, GarageBand can only be hosted inside Audiobus as output so you can route your audio from Audiobus into it. Others like Moebius Lab and apeMatrix can only be input sources. Some like AUM, Cubasis, Audio Evolution Mobile Studio, and modstep support input and output.