I found the answer to the "Significant Weather Alert"
Criteria is as follows:

"Significant Weather Alert" - Bulletin issued to provide information about thunderstorms causing any of the following problems:
Unusually frequent/intense cloud-to-ground lightning
Winds 40 to 50 mph
Hail smaller than pennies
Heavy rain causing significant visibility restrictions...especially near major highways during high traffic times... or ponding of water on roadways.

Product Identifiers - ___SPS___ or WWUS82
(Blank spaces are for the NWS Office Identifier~danielw)

This product is updated... as conditions warrant using another "Significant Weather Alert".
http://www.srh.noaa.gov/ffc/html/whatissps.shtml

So this is issued when a heavy Thunderstorm does NOT meet Severe Thunderstorm criteria~danielw

"Severe Thunderstorm Warning" - Bulletin issued when a severe thunderstorm has actually been observed by spotters or indicated on radar, and is occurring or imminent in the warning area.
A "severe" thunderstorm is one that produces:
Hail the size of pennies or larger,
or damaging winds (winds above 58 mph).
http://www.srh.noaa.gov/ffc/html/whatissvr.shtml