# String in APRS report

Here are two messages I received:

10:50:53R W2MEB-9>SX2X9U,W0BJ-1*,K0ATT-10*,WIDE2* <UI R Len=34>:vNN" dk/]"64}Monitoring 144.905=
10:50:58R WB9QPM-4>APBPQ1,AD9OV-8*,WIDE2* <UI Len=47>:!3924.30N/08848.72WI Shelbyville Igate EM59oj
`

What I'm asking about are the 6 characters after the > (the beacon "To" field). I've found that values starting with "A" indicate software version (in this example, APBPQ1 maps to "John G8BPQ Digipeater/IGate" from http://aprs.org/aprs11/tocalls.txt) by I can't find any resource that defines values starting with "S". Is there one somewhere? What does it mean?