Different heat range. Check this out
http://www.acdelco.com/html/pi_plugs_ident.htm .
I just put in what my (trusted) parts guy said to use!
Edit: I just went to that Delco site and used it to find their recommended plug - it's R44T. My TS is same plug but with extended tip. Pretty handy site; it lets you search for a wide range of AC parts for specific vehicles.