What's difference between AppMeasurement.js and at.js?

To implement Adobe Analytics, there's one guide mentioned:

https://experienceleague.adobe.com/docs/analytics/implementation/js/overview.html

This will use AppMeasurement.js

However, there also another link:

https://developer.adobe.com/target/implement/client-side/

This will use at.js

So my question is:

what's difference between AppMeasurement.js and at.js? which case should be depending on AppMeasurement? and at.js?

Or at.js is future, AppMeasurement.js is past?

Thanks,

Kent