Great comments about the evolutionary order. The only correction, is that Flag Keepers are not a membership level (unlike FC, FR and FB), but instead an elected body of 7 Flag Bearers who have an operational role within the DAO, which operates as an organizational unit.
I personally like the suggestion of the graphically representation that @Droste suggested, which may help with understanding the structure. And yes, membership is different from the organizational Flag Keepers unit. The membership levels give you governance rights (Flag Raiser and Flag Bearers only) including submitting proposals and voting in new membership (Flag Bearers only). The Flag Keepers are responsible for the signing of transactions on the multi-sig, appointing roles (like Discord moderators and InfoSec team), managing the DAO tools (i.e. admin rights to social media accounts, etc.).
The Transition Council wanted to simplify the DAO away from the former guilds and groups when we re-built the DAO structure. However as a Flag Bearer, if the community decides they want to add groups or units, the power of the Codex empowers the community for any changes.