Nikon DTM-502 Series Instruction Manual page 109

Once the function is assigned to the [USR] key,
every time you press the key on the BMS, the
selected function is directly called and activated.
3
• Changing the function assigned to the [USR] key
can also be done in [MENU] → [6.1sec-Keys] →
[3.USR]. (See p.3-129)
3-83
[USR1]