This is the first time I am noticing this symbol on this post: https://discuss.tchncs.de/post/63754269/26869571

On lemmy.world (original instance) it looks like this:

I know the [M] symbol means the user is a moderator of the community, but what does the red shield to the left of it mean? I don't seem to have that on the communities I am a moderator of, and wasn't able to find anything in the documentation.

It means the comment is “distinguished” (or marked as “speak as moderator”). It basically means that the user is posting officially, in their role as community moderator. On old reddit, this was marked by a green username background.

In my opinion, lemmy should also only show the [M] moderator icon when the comment is distinguished, but unfortunately that's not the case.

OK, thanks; how do I enable this myself in the communities where I'm a moderator? I don't see that option.

If you're using lemmy-ui (the default), it should be an option in the context menu after posting a comment. I don't think it's possible to do while you're still writing it.

Thanks, that worked.