Excuse me, what is the difference between SNAP-IDC-16 and SNAP-IDC-HT-16? Can you give me an example of the application of the two modules?

thank you!

Take a close look at the specs of the two;
The input resistance of the HT is a lot lower, 4k vs 44k.
This is then reflected in the ‘On Voltage’ the HT is 15vDC@3.5mA vs 10vDC@0.23mA

That’s about the guts of it. The HT is more leakage tolerant, if you have a prox switch that is going to leak a bit of current in its off state, the HT will be the one you need as that leakage would probably turn on the SNAP-IDC-16.

Here’s “switch prox” is the electromagnetic proximity switch, right?

Correct, and it is just an example. Any device that has some leakage voltage/current when it is off may not turn off the IDC-16.
Just check the specification sheet for the device. If the off state leakage is more than say half to one milliAmp, I would use the HT version.