I see there is a J1939 message for Engine Total Hours of operation. Would I be right in thinking that the Mhrs application would take this information and use it to store the total actual machine hours in UserDefinedVariable.MHrs.machineHours. Is there an example of this?

The j1939 message is used to recieve engine hours from another device, typically the ECM. The data would be stored in the usual format for J1939 variables (J1939.Engine.Engine Total Hours of Operation).

The Mhrs application is not related to this. There are 3 actions that are associated with the Mhrs application, increment, store and clear. The Increment will increment the engine hours variable by .1, the store saves the variable into a file and the clear will zero out the varialbe.

The intent of the Mhrs application is to allow some flexibilty around counting Mhrs, for example only incrementing Mhrs if the engine rpm was above 50. This prevents the engine hours from being incremented when the display is on, but the engine is not running.