Combat site anomalies are ranked in difficulty by 10 levels, these are:

(Faction) Burrow / Drone Collection (easiest)
(Faction) Hideaway / Drone Cluster
(Faction) Refuge / Drone Assembly
(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)

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

(Faction) Hidden (anomaly) [easiest]
(Faction) Forsaken (anomaly) [normal]
(Faction) Forlorn (anomaly) [hardest]