Unit test guidelines - rule or skill ?

I've been digging older posts about Rules vs. skills and when either should be used.

According to a post on the Cursor forums -

• Rules: permanent instructions like code style and architecture standards (short, under 500 lines)
• Skills: specialized domain knowledge for the agent if you’re on nightly

To me, it feels like under the hood, a rule that is applied intelligently by the agent vs. a Skill should behave pretty much the same.

In the context of "unit testing guidelines" - a set of project and team conventions for creating tests - which one should be used - a rule or a skill? and why?