On the surface flair on PieFed functions very similar to how it does on Reddit – on posts they’re community-specific tags that can be used to filter posts in a community. People can also add flair to themselves which is just a piece of text that appears next to their name whenever they make posts or comments in the community. This can be helpful for giving a hint about someone’s background, interests or expertise.

However PieFed is federated and there are copies of the communities on multiple servers (instances). The way to use ActivityPub to create and maintain those copies is described in FEP 1b12 which makes no mention of flair. I have made some minimal additions to that FEP, described in the link.