The forum officially takes the position that as long as you remain within the
Recommended Levels you don't need to worry about the CSI. This is in spite of the fact that if one were at the lowest end of those recommendations for pH, TA, CH and the highest for CYA that the CSI for SWG (3000 ppm salt) plaster pools would be at -0.5 or thereabouts depending on temperature and -0.6 if borates are used.
As for the meaning of the CSI, at 0 the water is saturated with calcium carbonate so it will neither form scale nor dissolve plaster. When the CSI is positive, it can form scale but in practice we don't see this occurring in pools until at least +0.7 except in saltwater chlorine generators at their hydrogen gas generation plate. In hot spas scaling may be seen at around +0.3. When the CSI is negative, then it can dissolve the calcium carbonate in plaster and grout surfaces. It is most critical to avoid negative CSI with young plaster since it is most susceptible to degradation when it is new. After it is hardened, it takes longer to dissolve. Plaster coupons were seen to release calcium at a CSI of -0.6 to -0.7 or so, but there is debate as to how negative is how bad when one is looking at trying to preserve plaster for many years, even a decade or more. A low pH is most detrimental to plaster and it may accelerate the dissolving of it for any negative CSI.
Technically speaking, a CSI of -0.3 has half the amount of calcium or carbonate or their product of concentrations compared to being saturated. -0.6 has one-fourth the amount of saturation. -0.9 has one-eighth the amount. -1.0 has one-tenth the amount. So even though these sound like small negative numbers, they are a logarithmic scale so even -0.3 is much bigger than it sounds -- again, half the amount of saturation.