6/8, 9/8, 12/8... are 'compound' time signatures. This means that the beat is a dotted quarter note, (or a dotted crotchet). 2/4, 3/4, 4/4 are 'simple' time signatures, where the beat is a quarter note (crotchet).

Thus if a piece is in 6/8, there will be six eighth notes (quavers) in the bar, divided into two beats of a dotted quarter note. In 9/8 there will be three compound beats, in 12/8 there will be four, etc

You can divide a simple time signature into triplets, and this is how style programming works. In the case of 12/8 styles (and technically with 6/8) a 4/4 time signature is used, and each quarter note beat is divided into 3 eighth note triplets. The result is then exactly the same as 12/8, ie: four pulses divided into three.

The BIG and significant difference between 6/8 and 12/8 on an arranger keyboard is that the fills in the 12/8 styles would seem to last for 2 measures (bars) of 6/8. But otherwise how would you tell the difference? One bar of 12/8 would just seem like 2 bars of 6/8. However, if you think of the "average" 50's Rock Ballad, the cycle of the style often fits more with 4 pulses to the measure than with two.

In the case of a 6/8 March, where there is a definite two beat feel, these are still programmed in most arrangers in 4/4 (ie 12/8) and in this case you really get two measures for the price of one. Also the fact that fills last in effect for two measures seems to work better at a faster tempo (ie 120-ish).

Whatever, what you get on most arrangers is 12/8, created using 4/4 and each simple beat divided into eighth note triplets.

It is either actually 12/8, or in the case of a style which is really 6/8 you just get two real measures for what is displayed as one measure on the instrument, and the most obvious musical result of this is that fills seem to be two measures long.