These four letter codes represent the "pools" that locos are allocated to for operational reasons to denote their intended use.
WFMU is the DB Schenker "Fleet Management" pool: DB locos are regularly transferred between the different traffic specific DB business sectors (Energy, construction, etc), and so the Fleet Management pool acts as a catch all for locos that are not specifically allocated to one or the other.
WBEN - DB Schenker class 66s used in France by Euro Cargo Rail.
WNWX - DB Schenker main line locomotives for major repairs.
WNTR - DB Schenker locomotives: Tactical reserve fleet
WNTS - DB Schenker locomotives: Tactical reserve fleet, stored unserviceable.
WNXX - DB Schenker locomotives in store.