Shrinkage multiplier is really used. If you create a material with .98 (2% post cure shrinkage), it will increase the image and the z steps such that it's printed larger. (and hopefully shrinks to correct size after post cure).
MDM, This is a factor that comes from the configuration file, yes it's the projector multiplier. The line in the config file should be changed to "Machine Dependent Multiplier" (and will be at some point). It's meant to adjust for more than just the projector.
XY is for scaling the image to compensate for errors made in the Projector Calibration (done using the grid). The only way to use it effectively is to do the best possible Projector Calibration, print a test object, and then measure the actual print before post curing. The multiplier would be the object's designed dimension divided by the object's actual measured dimension.
On any given layer, when we expose a pixel, there is either a pixel "above" it already formed and we need to cure long enough that we ensure we adhere to it (ie, supported) or there is no pixel above it and so we want to cure long enough to form the layer at the desired, sliced, thickness. The normal calculated exposure times ensure we "stick" to the above pixels. As a standard, for unsupported pixels, we expose only 80% of that time, to reduce the z-bleed issue.
Measure Twice, Cut Once.