Summary: The Default Address is the address specified in the NMRA DCC Standards for multifunction decoders.
If a multifunction decoder is reset, all Configuration Variables will be restored to their factory default configuration, including the primary and extended address. The NMRA DCC Standard specifies a retail multifunction decoder use a primary address of 3.
Some decoders have multiple reset procedures, to allow a portion of the decoder's CVs to be restored to their default values.
After a decoder reset, the primary address is set to its default value. Contrary to popular belief, any valid address can be the default address.
The Default Primary Address of 3 applies to multifunction decoders sold at retail only. 
Setting the Address
- S-9.2.2: The default value for this Configuration Variable is 3, if the decoder is not installed in a locomotive or other unit when shipped from the manufacturer.
- It is also permissible for the installer, dealer or locomotive manufacturer to set the default address.