Anomalies are ranked in difficulty by 10 levels, these are:

(Faction) Burrow / Drone Collection (lowest)
(Faction) Hideaway / Drone Cluster
(Faction) Refuge / Drone Gathering
(Faction) Den / Drone Gathering
(Faction) Yard / Drone Surveillance
(Faction) Rally Point / Drone Menagerie
(Faction) Port / Drone Herd
(Faction) Hub / Drone Squad
(Faction) Haven / Drone Patrol
(Faction) Sanctum / Drone Horde (hardest)

A particular anomaly may be further sub-divided into 3 more categories:

Blood Raider Hidden Hub (easiest)
Blood Raider Forsaken Hub
Blood Raider Forlorn Hub (hardest)