Beckhoff AX8000 Series Operating Instructions Manual page 48

Technical description
Safety
48 ───
Information on the meaning of the different symbols on the display
of the safety axis module is given below:
S = Safety
Safety symbol lights up green:
No safety error.
Safety symbol lights up red:
The axis is in state "STO".
Symbols of EtherCAT, axis module and safety light up green:
The safety axis module is in the normal operating state.
These symbols are only visible when the two-channel safety axis
module has reached its operating state.
AX8000 Version: 1.1