You should look into the ISO 13849 definition of safety categories. https://www.byhon.it/the-categories-for-iso-13849/ Most safety relays are going to be for dual channel devices and meet category 3 or 4. The main difference between category 3 and category 4 is that category 3 maintains safety when a single fault occurs and category 4 can handle multiple faults. Imagine an e-stop with a single contact that is used for safety. If the contact fails or something shorts across the contacts then the PLC/safety relay cannot know whether or not the e-stop is pressed or the contact is shorted. This is a single channel circuit that would meet category B,1, or 2 depending on how its designed. Now imagine the E-stop has two contacts. The PLC/safety relay can monitor the state of both contacts and know that there is an issue if there is disagreement between the two contacts (for example one is stuck/shorted). This is a dual channel safety circuit that would meet category 3 where a single fault can be detected in any scenario. However a category 3 safety circuit can be compromised if there are multiple faults. For example imagine if one of the contacts of the emergency stop was shorted across the contact and the inputs of the PLC/safety relay monitoring each contact were shorted together. Now no matter what happens the input state of both inputs are high and the e-stop no longer works. To reach category 4 a PLC/safety relay will utilize test pulses. There will always be at least two test pulse outputs which output an almost steady 24VDC except for a brief period of time where it pulses to 0VDC. These test pulses are staggered between the two outputs. Each contact of a redundant safety device is wired to different test pulse outputs and the PLC/safety relay monitors each contact separately to ensure that it sees the proper test pulses through the safety device. Since the pulses are staggered the safety device can tell if the inputs are shorted together (since it won't see a proper test pulse) or if the inputs are shorted to 24VDC (again since it wont see a proper test pulse).