What does the social security card sk mean at the beginning?

The first letter of the social security card number represents the province.

This English letter generally represents the letters of social security cards in various regions, just like the letters in front of license plates in various regions are different, which is the same thing.

Therefore, the s in front of the social security card number indicates that the social security card numbers in this area all start with s, and the region to which these social security cards belong is determined by the initial letter of each region.