What is the difference between the WSUSnn IDs used in convective SIGMETs?
The WSUS31 is an WMO ICAO communications header. It is not a SIGMET category, it just identifies the basic item and source, in this case WSUS31 is weather-SIGMET united states, 3x is for regional distribution (eg the Americas) x1, x2, x3 are the east, central, and west SIGMET areas in the contiguous USA[CONUS].
Another example: An AIRMET for the far west area would be WAUS46. AIRMETs have 6 areas in the CONUS and 3 in Alaska, 4x means national distribution.
Distribution codes

0x,1x global
2x,3x regional
4x,8x national and bilateral
9x reserved

https://www.weather.gov/tg/awips
https://www.weather.gov/tg/headef
(sections 3.4.1 and 4.3.1) https://www.icao.int/EURNAT/EUR%20and%20NAT%20Documents/EUR%20Documents/EUR%20Documents/014%20-%20EUR%20SIGMET%20and%20AIRMET%20Guide/EUR%20Doc%2014%20%20(EN)%20-%20Edition%202,%202010%20-%20rev%2029Oct2019.pdf