I created a couple of dummy projects in a repo, and trying to see how the multiple lockfiles feature works. Question: do I need to use resolve="something" in each and every BUILD file in the projects? I could not find a way to specify it for the whole project, and I get errors if it's not everywhere.

Yes you will have to until 2.13. Check defaults which is introduced in 2.14.0 (still in beta though) to handle such use cases.

Great, thank you!

One resolve is set as the default, so you don't have to set it on targets you want to use the default

2.14 is now at a release candidate, so may be worth trying so you can use

__defaults__

. This PR improves the docs for it https://github.com/pantsbuild/pants/pull/16377 We're hoping to get out a stable 2.14 release in the next ~2 weeks