Torpedo's proximity fuse?

When I look through the code of some torpedoes , I found out that some of them(SET-40,SET-72,ect) have proximity fuse that is only sensitive to shells.But I can’t make it work in any way.So does anyone know what is this for?
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Maybe because they were originally anti-submarine torpedo and there are not submarines so react against torpedoes and shell near water.

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More than likely hit things like mines or depth charges