Nephilim is the lowest Rank of Angel, but they do have a set structure.
Which goes as follows Nephilim -> Cherub -> Cherubim -> Seraphim -> Arch Angel -> Celestine
1st Generation Angels are set in their Rank as they were created. There are very, very few exceptions to this. Lucifer, for example, began as an Arch Angel and now holds Celestine status. Azazel in his rebellion hoped for the same and he and his followers were stripped of all physical form for their trouble. Rumors/Worries are that the Immolo may fall as Gabriel seems to be studying the mechanism by which those of the 2nd Generation are allowed to Ascend through the Ranks.
The higher the Angel’s Rank, the more powerful it is. The Halo burns brighter, and it can have higher maximum stats. For the most common Angels, wings are the biggest identifier of its Rank. Nephilim are usually wingless or have very small, vestigial wings. Cherubs have small wings, too small in appearance to fly but they still can. Cherubim have a full sized pair of wings. Seraphim have 2 pair of full size wings. Arch Angels have 2 pair but can look like anything the choose… given that there are so few of this Rank still alive, they are all known by name anyway.
Poena Cherubs are somewhat affectionately known as “Imps”.
Other types of Angels have different appearences in while in Halo. Danu gain more Fae-like features, Nyx become more like living shadows, Coutyl turn into winged snakes and may have multiple sets of wings or look more draconic, and Kali gain extra sets of arms.
Nephilim = 5
Cherub = 6
Cherubim = 7
Seraphim = 8
Arch Angel = 9
Celestine = 10