Difference of a few projections in SNAP

When I handle Sentinel-1 image in RTC in SNAP software. I found there are similar projections, these projections include Polar Stereographic,Polar Stereographic (variant B),Stereographic North Pole and Stereographic.I want to know the difference between them.
Thanks.

I originally think the projection in SNAP is specific,then I search for these projection in Google.I have found their information.
Thanks.

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