A sub tropical system basically means we have a hybrid storm. It looks like a normal area of low pressure that moves throughout the natiion's midsection, but acts like a warm core tropical low. Its a category for those storms that quite don't fit the typical definition of a Tropical Cyclone.
Here's how NHC defines it:
A sub-tropical cyclone is a low-pressure system existing in the tropical or subtropical latitudes (anywhere from the equator to about 50°N) that has characteristics of both tropical cyclones and mid-latitude (or extratropical) cyclones. Therefore, many of these cyclones exist in a weak to moderate horizontal temperature gradient region (like mid-latitude cyclones), but also receive much of their energy from convective clouds (like tropical cyclones). Often, these storms have a radius of maximum winds which is farther out (on the order of 100-200 km [60-125 miles] from the center) than what is observed for purely "tropical" systems. Additionally, the maximum sustained winds for sub-tropical cyclones have not been observed to be stronger than about 33 m/s (64 kts, 74 mph)).
Many times these subtropical storms transform into true tropical cyclones. A recent example is the Atlantic basin's Hurricane Florence in November 1994 which began as a subtropical cyclone before becoming fully tropical. Note there has been at least one occurrence of tropical cyclones transforming into a subtropical storm (e.g. Atlantic basin storm 8 in 1973).
Subtropical cyclones in the Atlantic basin are classified by the maximum sustained surface winds:
less than 18 m/s (34 kts, 39 mph) - "subtropical depression",
greater than or equal to 18 m/s (34 kts, 39 mph) - "subtropical storm"
Prior to 2002 subtropical storms were not given names, but the Tropical Prediction Center issued forecasts and warnings on them similar to those for tropical cyclones. Since 2003 they are given names from the tropical cyclone list.
For more information see Penn State University's write up on the Subtropical Cyclones.