There are per-user flairs and per-community flairs, whereas hashtags are per post and can connect together posts from otherwise desperate communities, or now it can also help filter posts inside the same community. Also, community flairs are usually limited to being pre-defined by a moderator whereas hashtags come from inside the posts submitted to the community.

In !movies@piefed.social, we have 4 flairs

• Trailer/Poster
• Movie thread
• Discussion

Hashtags would be more specific, like actors name, movies name, genre etc.