What is the difference between the WSUSnn IDs used in convective SIGMETs? When NOAA issues a convective SIGMET, it falls under one of three bulletin IDs: WSUS31, WSUS32, or WSUS33. Below are three examples from the current forecast.
WSUS31:
WSUS31 KKCI 020455 
SIGE  
CONVECTIVE SIGMET 24E 
VALID UNTIL 0655Z 
PA 
FROM 30NNW JST-20SE EWC 
LINE TS 25 NM WIDE MOV FROM 24025KT. TOPS TO FL340. 
 
OUTLOOK VALID 020655-021055 
AREA 1...FROM 130SSE ILM-210ENE TRV-TRV-SAV-130SSE ILM 
WST ISSUANCES POSS LT IN PD. REFER TO MOST RECENT ACUS01 KWNS 
FROM STORM 
PREDICTION CENTER FOR SYNOPSIS AND METEOROLOGICAL DETAILS. 
 
AREA 2...FROM 70NNE 
ASP-ECK-DXO-CLE-BUF-SYR-BKW-VXV-MSL-TTH-TVC-70NNE ASP 
WST ISSUANCES POSS. REFER TO MOST RECENT ACUS01 KWNS FROM STORM 
PREDICTION CENTER FOR SYNOPSIS AND METEOROLOGICAL DETAILS.

WSUS32:
WSUS32 KKCI 020455 
SIGC  
CONVECTIVE SIGMET 30C 
VALID UNTIL 0655Z 
MI WI LS 
FROM 30SSE YQT-40SSE DLH 
LINE TS 30 NM WIDE MOV FROM 27050KT. TOPS TO FL310. 
 
OUTLOOK VALID 020655-021055 
FROM TTH-MSL-ELD-LFK-DLF-TCC-GCK-RZC-TTH 
WST ISSUANCES EXPD. REFER TO MOST RECENT ACUS01 KWNS FROM STORM 
PREDICTION CENTER FOR SYNOPSIS AND METEOROLOGICAL DETAILS.

WSUS33:
WSUS33 KKCI 020455 
SIGW  
CONVECTIVE SIGMET...NONE 
 
OUTLOOK VALID 020655-021055 
TS ARE NOT EXPD TO REQUIRE WST ISSUANCES.
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