SN?

I too wanted to know and this is what I found:

In a book called "Mustang, the Next Generation" (McClurg), SN-95 was listed as the internal Ford designation for the FOX-4 (or revised Fox) platform. The 'S' was for 'speciality', Ford-speak for the Mustang's market segment, and 'N' for North America, the geography indicator of the people responsible for its engineering. The '95' was a program sequence number.
 
the SN95's are the ones after the Foxes (our cars)...they ran from 1994 until 1998. If you wanna know what one looks like then go to the SN95 forum and look at some pics...you've seen them, they're not a special model or anything uncommon

Alternately, SN might stand for "StangNet" or "Screen Name."
 
SN95 is the chassis designation given to 94-04 Mustangs.

It's a modified Fox chassis, but different enough to Ford that they gave it a modern chassis designation.

It means:
S = specialty
N= North America
95 = 95th chassis certified
 
All vehicles leveraged from the fox platform:

FORD FAIRMONT 1978-83 (105.5" wheelbase; 2-door, 4-door, station wagon)
MERCURY ZEPHYR 1978-83 (105.5" wheelbase; Z-7 coupe, Villager wagon)
FORD FAIRMONT FUTURA 1979-82 (105.5" wheelbase; 2-door only)
FORD MUSTANG 1979-93 (100.4" wheelbase; 3-door hatchback, 2-door coupe)
MERCURY CAPRI 1979-86 (100.4" wheelbase; 3-door hatchback, 2-door coupe)
FORD GRANADA 1980-82 (105.5" wheelbase; 2-door, 4-door, station wagon)
MERCURY COUGAR 1980-82 (108.2" wheelbase; 2-door, 4-door, station wagon)
FORD THUNDERBIRD 1980-82 (108.2" wheelbase; 2-door only)
FORD LTD 1982-85 (105.6" wheelbase; 4-door, station wagon)
MERCURY MARQUIS 1982-85 (105.6" wheelbase; 4-door, station wagon)
LINCOLN CONTINENTAL 1982-87 (112.0" wheelbase; 2-door only)
MERCURY COUGAR 1983-86 (103.8" wheelbase; 2-door only)
FORD THUNDERBIRD 1983-86 (103.8" wheelbase; 2-door only)
LINCOLN MARK VII 1984-92 (112.0" wheelbase; 2-door only)
MERCURY COUGAR 1987-88 (104.2" wheelbase; 2-door only)
FORD THUNDERBIRD 1987-88 (104.2" wheelbase; 2-door only)