IEC 1131 61131 permissive schemes

Hello,
I am reading an spec that literally says: ". The provision of control permissive schemes compliant with the IEC 1131 standard is
encouraged."
I have tried to find-out about 1131 permissive schemes and treatment but I have not found anything.
Does anybody know what this sentence refer to and where to find some info??
Thanks a lot.
Alex.
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