That’s “the question”. I will do my best to bluff through it.

Engineering team size: you have teams large enough (~3-4) to own and iterate on a subset of related functionality for the long term.

Tooling: you have a builder tools type team that provides a tooling and observability happy path.

Traffic scale: you have functionality that operates at 2 or more order of magnitude higher that the rest of your application.

Decoupled: you have functionality that can be decoupled from your main app and, most importantly, isn’t required for your main app’s uptime. Like a search service or something.

A combination of the above.