Comment by InclinedPlane

It's a feature flag, but it's one that user's don't have easy access to. Pretty much every big software company does this, because it's a very effective way of testing new code. I know MS, Amazon, and Google all have tooling designed to facilitate just this sort of thing.

Let's say you've rewritten core parts of some system, but you've also added new features that didn't exist before. Often times such changes are inextricably bound together. On the one hand you may want some users to test out the new code to make sure it works correctly, but on the other hand the new features you've added may not be fully "baked" so you want to restrict their use. That's where secret feature flags come in.