I know CPM is cargo pallet message and LDM is load distribution message, but what's the difference between them ?
Purpose of the LDM is for the load-planner on the next leg of a multi-sector flight knows the figures for the next leg.
Example.. if the flight is routed SIN-DXB-LHR, the LDM from SIN will be broken up into 2 parts, one for SIN-DXB and and one for SIN-LHR. The loadmaster in DXB will use the data in the SIN-LHR part of the LDM telex for his load-sheet and then add in the load joining from DXB.
The CPM is used for aircraft where ULDs are loaded. This will give the loaders info as to which containers are for which destination and also which ULDs contain bags and which contain cargo/mails etc.
A CPM is mandatory for all sectors of ULD equipped aircrafts. An LDM is required for all sectors on all aircraft types except the last sector for an aircraft with ULDs as the CPM is sufficient. As aircraft with only bulk does not have any CPM, the LDM, while not required by most airlines, is still a way to inform the next station of what to expect on the incoming flight.