These descriptions aren’t really valid for this, they are generic ones from all modules pages but not every module uses them the same way. In this case it’s just a relative ranking of long-term projects in the sculpt module.
This is the difficulty with having an open development process. If you’re just talking between developers within a company everyone knows what is meant by this, and for any external communication you take extra care. But if it’s all in the open people will nitpick things like this.
We could rank everything as high priority but that would be meaningless. The point is just that core multires improvements and performance improvements were decided to be higher priority, before adding more features like layering. Adding those while the core architecture is changing would not work well.