Rest api etag syntax

Is there any further reading or examples of etag use in rest api calls?
The api docs mention it in a very brief manner without in depth elaboration.
How to pass to api endpoints within header? How to fetch it?
Are weak validators accepted?
What is Server response in cases If-Match / If-None-Match?
The related HTTP 304 and 412 are apparently absent in Api statues codes.

I don’t believe we implement no-modified (304) but we do 412.

For that, stuff the value you get from the ETag header into the If-Match header and LXD will check it. I’m currently doing API documentation work so I’ll make sure that endpoints that accept If-Match are documented to return 412 if it doesn’t match.

Just confirmed that our new entries in do have 412 listed where it’s possible.

Indeed, PUT and PATCH seem to reply with 412 when condition unmet.
Yet would appreciate if you fire up once more another legendary blog/howto with some practical examples in this regard.