WMD is a title unit in the WebMoney Transfer system that is WMZ equivalent for credit transactions kept track of in purses of D type.
A purse of the D type is used to keep record of received credits.
Purse of the D type can be created only once and cannot be deleted. You can create a purse of the D type only if you have a passport no lower than formal.
WMD title units are actually promissory notes (obligations to transfer a respective amount of WMZ title units by a certain date) to the creditor of the system user who received a credit.
The number of units in a WMD purse corresponds to the total amount of WMZ that must be transferred to the owner of WMD purse.