Chauvin Arnoux C.A 5233 Notice De Fonctionnement page 49

English
The
3.2.6
This key is used to display the value relative to a reference
stored when the key was pressed.
For example, if the value stored when the key was pressed =
10V and the current value is 11.5V, the display in relative mode
will be 11.5 - 10 = 1.5V.
Note: the Auto-Range mode is deactivated.
Each press of
this length...
short
long
(> 2 sec)
49 - 139
key
 to display the relative value ;
 to exit from the
To deactivate automatic switching off.
...serves to
mode.