When to use # or not in header attributes?

Please let me give you this header snippet from an org-roam file.

#+title: MyMeta
#+filetags: Meta
+date: [2021-11-20 Sa 21:11]

This example is from a youtube video. And my question is why isn’t there an # in front of +date? Are there rules when to use # and when not in the header?

The first two lines are interpreted by Org mode, the third line is most likely a convention by the video author

Yeah, the third line is probably author convention, it’s not recognized specially by Org-mode in any way.