When in an inflow (updraft) area, scud clouds tend to rise and may exhibit lateral movement ranging from very little to substantial.

Pannus clouds are formed as the warmer (and often more moist) updraft of a thunderstorm lifts the relatively warm air near the surface.

These clouds condense as the warm, moist air saturates through ascent and is pushed outward from the storm.

[2] Pannus clouds may also form when an updraft ingests precipitation-cooled air from the downdraft.

The difference is determinable by observing the presence or absence of rotation (not just movement) of the scud clouds.