I love it when the right questions are asked.

The saturation index, sometimes called the Langeliers Index, is a formula that takes the following into consideration to see if your water is perfectly balanced, has a tendency for scale formation, or a tendency for corrosion.
pH (7.2 - 7.8)
Total Alkalinity (80 - 120 ppm)
Calcium Hardness (150 - 500 ppm)
Water Temperature
TDS (total dissolved solids - which should be high as a result of the added salt)

The test values are given an equivalent "factor" value and the numbers are plugged into the Langeliers Index formula.
There's an acceptable range for these parameters to fall within and in most cases, it is though that as long as these parameters are within this range, all is well. The fact is that if one or more are on the higher end of the range (or above), there is a scaling tendency. With a Salt Chlorine Generator, the scale will development in the cell first. Then you may see some scale precipitate on your tile line or on the pool surface if really bad.
If one or more are on the low end, there is a corrosive tendency, in which you may see damage to your handrails, ladders, light rings, or heat exchangers.
If the formula falls within + - 0.3, you're within balance. Greater than 0.3 means scaling tendency. Less than - 0.3 and you're corrosive. The greater the difference the greater the tendency for scale or corrosion.

If the pool store is not familiar with this, see if they have the TAYLOR K-2005 test kit. In it is a wheel called the WATERGRAM, that allows you to calculate the water balance easier than the Langeliers Index...
or report the test results here and we can help decipher the test results.

Congrats on selecting a Salt system for your pool.

Now, what are you going to do with your free time not spent maintaining your water chemistry? : )