Rod Stiffness and Action - SA#
Stiffness Index
+1 Rod is best with stated line weight or can be used with one line weight heavier without feeling too overlined.
0 Rod is best with stated line weight but will cast one weight either up or down without feeling too overlined or underlined.
-1 Rod is best with stated line weight or can be used with one line weight lighter without feeling too underlined.
Action Index
T Primary flex is in the tip or the upper 1/2 to 1/3 of the rod.
M Primary flex is in the mid section or middle 1/3 of the rod.
B Primary flex is in the butt or lower 1/3 of the rod.
The Stiffness Index number ( +1, 0 or -1 ) is used pretty much as shown.  The Action Index letters are sometimes combined to describe in better detail how a rod bends during casting and on occasion also using upper and lower case letters; a lower case letter designating less of a particular bend characteristic than a capital letter. 

A good example would be the Stiffness/Action number for my LM Series taper which is +1Mt and would translate to +1 for the stiffness and a mostly mid-flex (upper case 'M') but still exhibits some softening of the tip (lower case 't').  Another example would be the Clark Gierach/Best taper which I've given an SA# of +1mT and would designate a rod with +1 stiffness that is somewhat mid-flex (lower case 'm') and includes a more significant bend in the tip (upper case 'T').