Boss BR-1200CD Owner's Manual page 57

Selecting drums to act as a guide for recording
At the time of purchase, the BR-1200CD contains drum patterns only in Preset patterns (P001–). User patterns (U001 –)
and Song (S001–) are provided for you to freely create your own drum patterns, and these patterns are initially empty.
If you select "U" or "S" at this time the BR-1200CD will not be able to play back a rhythm track.
Even while the drum pattern is being played, you can repeat Step 5 above to select a new pattern. In such a case,
playback of the newly selected drum pattern will start as soon as the previously selected drum pattern has ended.
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