Specifies how long the pattern should unmute for, measured in 16th notes (irrespective of a pattern's BPM division setting!). If 'Mu.Ln' + 'UM.Ln' does not the equal pattern length (Pt.Ln), interesting polymeters can eventuate. Unmuting happens after the amount of muting steps specified by 'Mu.Ln' have elapsed.
Note that the time when 'Mu.Ln' begins, can be offset to occur later or earlier using the 'Mu.Of' (mute offset) parameter. This means that it is possible to change 'Mu.Of' in such a way that a pattern starts off unmuted instead; simply set 'Mu.Of' equal to negative 'Mu.Ln'.