A B777-200 Pilot reports that during taxi out the DOOR FWD CARGO caution message appeared intermittently on the upper EICAS display. The DOOR FWD CARGO IND status message on the status page was already deferred. Pilot noted they found the wording of the MEL Chapter 52 to be confusing and inadequate for the situation they had on taxi-out.

Narrative

During flight planning; we noted that our aircraft had a deferred item for the Door FWD Cargo IND (indication) inoperative MEL Chapter 52-00. During preflight; after cargo loading; Maintenance advised that the forward cargo door was verified to be closed; latched; and locked. We noted the DOOR FWD CARGO IND status message on the status page. There was no caution message on the upper EICAS and the synoptic showed the affected door as 'Closed.' During taxi out; the DOOR FWD CARGO caution message appeared intermittently on the upper EICAS display. This was a change from what we had seen at the gate. We also read from the MEL placard a note stating: If a door alert level message is displayed without the associated status message; operation of the airplane is not authorized. We called Dispatch and Maintenance Control to discuss the indications and verify that the actual condition of the airplane met the intended conditions of the MEL deferral. Maintenance Control advised that we return to a gate to verify again that the door was properly closed/latched/locked. While taxiing in; we noted that the Flight Manual procedure for DOOR FWD CARGO required descent to a low altitude to reduce differential pressure on the door. Obviously; we could not comply with the Flight Manual procedure and also fly to [destination]. So we needed to discuss amongst ourselves; together with Maintenance Control; what the MEL deferral intended that we were supposed to do in the event the DOOR FWD CARGO caution message occurred en route.After lengthy discussions; we determined that the airplane met the intended conditions of the MEL. Maintenance Control further confirmed that; given the presence of the DOOR FWD CARGO IND status message; the DOOR FWD CARGO caution message should be considered an erroneous indication -- one aspect of the inoperative Door Indication--and thus the Flight Manual/Emergency Checklists for that condition could be overridden. After confirming these things; we departed. This report is being written because we found the wording of MEL Chapter 52-00 to be confusing and inadequate for the situation we had.First; the MEL card should be changed to state clearly the fact that the DOOR FWD CARGO caution message might be displayed.Second; the card should state clearly that; if the DOOR FWD CARGO caution message is displayed; the airplane may; or may not be; dispatchable depending on whether or not the DOOR FWD CARGO IND status message is present. If the caution message is present and the status message is NOT present; the airplane must NOT be operated until the message is resolved.Third; we note that MEL Chapter 52-16-00 for Cabin Door Indications Inoperative includes a note that says that a cabin door caution message; without the associated door indication status message; may result in a CONFIG DOOR warning during takeoff roll. We assume that the same system logic exists for the cargo door indications. If the system logic is the same; the cargo door indication MEL Chapter should be revised to use the better wording found on the cabin door indication MEL Chapter.Finally; the MEL card should be revised to state clearly that if the DOOR FWD CARGO caution message occurs in flight; the appropriate action depends on whether or not the DOOR FWD CARGO IND status message is present. If the status message is present; the caution message may be considered erroneous; and Flight Manual/Emergency Checklists overridden. However; if the status message is NOT present; the caution message must be considered accurate and Flight Manual/Emergency Checklists must be accomplished.