The Ainur are the immortals of MUME. They do not take part of playing the game, but shape the world and keeps it running.
The Ainur are divided into groups, each of them divided into several smaller groups.
Newly created gods join the ranks of plain Maiar, also called boardreaders, since what they can do mostly consists of reading boards and handing out newbie kits.
Helping with fixing typoes, trying out creating a test-zone and various other tasks promotes an Ainur to become a Cartographer. Getting your own zone to build also does this.
When your zone opens, the builder can be promoted to Wright.
Higher levels can be applied for, and if an opening exist a Wright can be recruited and trained.
The Valar levels Mudller and Architect differ in level, but not in power. They are practically the same level but with different job descriptions. Mudllers write mudlle-code and Architects supervise builders.
Similarly the Aratar level is only one level, with only one Arata per job. One Arata of Code, one of Zones, etc. All Aratar are only seen with the acronym A, no extra-letter is visible. Who is acting in which job isn't really important to the public, but ask any Ainur and they'll probably know the answer.
There are Aratar without this precise job description too, but they have assignments and responsibilities (just not in plainsight for mortals).