Alesis ADAT-XT20 Reference Manual page 79

Chapter 4: Basic Operations
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points represent the same actual (physical) position on tape, regardless
of whether the display is in ABS or REL mode.
The [LOCATE 0] button is also used to set the relative zero time.
When this button is pressed while holding [SET LOCATE], the
relative tape counter is reset to zero (0:00:00.00). Once this occurs, you
have changed the Relative Time zero point, and the display will
automatically change to Relative Time mode (REL icon lit).
ADAT XT20 Reference Manual