Some 8-bit PIC® devices have locations outside the addressable memory area that can be used for storing program information, such as an ID number.
The config pragma is used to place data into these locations by using a special register name. The pragma is used as follows.
where x is the number (position) of the ID location, and value is the nibble or byte which is to be positioned into that ID location.
The size of each ID location varies from device to device. See the device data sheet for more information.
Refer the device datasheet for programming ID Location, and size of each ID location (nibble wide / byte wide).