https://www.w3.org/policies/process/drafts/#Note has

Some Notes are developed through successive Note Drafts before publication as a full Notes, while others are published directly as a Note. There are few formal requirements to publish a document as a Note or Note Draft, and they have no standing as a recommendation of W3C but are simply documents preserved for historical reference.

When groups are advancing a document toward being a Note, they're often unsure about when it's appropriate to publish as a Note Draft, which causes them to waste time discussing what a Note Draft signifies.

My sense of the difference is that Notes have WG consensus about their contents, while Note Drafts are just documents that the group has agreed to use as the basis for some work toward a Note. I support this by referring to the definition of Working Drafts:

Working Drafts do not necessarily represent a consensus of the Working Group with respect to their content, and do not imply any endorsement by W3C or its members beyond agreement to work on a general area of technology. Nevertheless the Working Group decided to adopt the Working Draft as the basis for their work at the time of adoption.

However, there's no necessary connection between the meaning of a Working Draft and a Note Draft, so it would be nice for the process to give a meaning for Note Drafts too.