Case Control

Am asking this on behalf of a friend, if that's ok.
Dwp have told them they are under Case Control.
Anyone shared any light as to what it means please?.Kind regards.
P.

Gordon you're back, how wonderful.

All I know is that it involves IR ESA. A change was reported by claimant over phone. Quite an interrogation ensued, followed by them being told 'case control'. Letter followed by what seems to be reduction in ESA to what I think to be ESA (c). Their PIP reassessment form was sent off a few months ago & in last couple of days text arrived saying they're now looking at PIP. I suggested that perhaps it was a standard text letting people know that their forms now being looked at.! Friend asked CAB what Case Control was? Reply given was to comply with all questions and requests for paperwork from DWP! Might it be that they are being investigated & monies reduced in meantime? Can they remove the IR component if not enough NIs were involved?
Kind regards
P