What does the specification, "Signal supply rating 15 VDC, 30 mA" mean?

The input types are non-voltage contact and open collector. Hence, it is not necessary to input a voltage to pulse input. This specification shows the withstand voltage and current capacity determined for contacts or transducers. In other words, a contact or transducer is needed that does not short even when 15 V is applied when OFF, and that does not short even when 30 mA flows when ON. Actually, at 5 V, 2.8 mA–3.8 mA, it is ON.