Siemens SIMATIC S5 Manual page 60

Zero offsets executed in a subroutine are not cleared following the return to the main program.
They are only reset on completion of the main program.
A zero offset changes the limits of the traversing range, the reference point and the actual posi-
tion value according to the value of the offset, With a positive zero offset, the zero point of the
coordinate system is displaced in a positive direction, i.e. the individual points on the axis have a
more negative value. A negative zero offset has the opposite effect.
"zero offset absolute or relative" (=> Sections 4,3.5 or 4,3.6 "Zero Offset Absolute or Relative"),
are not changed.
A statement containing one of the G-functions G54-G57 executes a relative zero offset.
The following example contains both types of zero offset, After stipulating the coordinate system
a relative zero offset 10 mm forwards is executed.
Mode BA 13
(zero offset relative)
The actual position value is displaced from O mm to -10 mm. Following this, a traversing move-
ment to point O mm is executed.
Parameter approach
Parameter 10000 ~m
Parameter
O mm
Machining Programs and their Structure
Start command,
Command forward
2 - 4 3