What i have seen across multiple projects is that when the ACT prefix is used for a microflow it means that it is being called from a button on a page. So not every microflow within your app should have the ACT prefix. Only the ways that are being called directly from a button. This also means that only these microflows need to have roles configured for it.
It is a naming convention that Mendix recommends.
You can see it here https://docs.mendix.com/refguide/dev-best-practices/#343-page-based-microflows
Using Prefixes can easily understand its purpose by looking at the microflow name itself
Check out the Best Practices: https://docs.mendix.com/refguide/dev-best-practices/