This is an excellent question. Fraxel has a patent protected technology where by the handpiece used by the technician, nurse, or doctor can roll over the skin delivering approximately 2000 microthermal zones (MTZs) per square centimeter. This number is an average and could vary depending on how the user programmed the laser's density setting. In general, the doctor will program the power, density, and number of passes into the laser and the laser will calculate how densely to space apart the microthermal zones. The power is measured in Joules and determines how deeply the laser penetrates. Fraxel is very powerful and can penetrate into the deepest part of the skin (reticular dermis) making it very effective. The density depends on the number of proposed passes and ranges from 10% to 70% coverage. The 20% number you have been told is an average treatment density using average settings. 20% is a very standard treatment and well within standard of care.
In general, most users will do 4 passes (2 horizontal and 2 vertical) with a 10% overlap between passes. The laser will calculate how densely to space the microthermal zones depending on the proposed number of passes and the desired density. The treated skin will therefore be a computed average for coverage and does not guarantee that the laser will not treat an area that has already been treated. However, this average is very precise and if the doctor or nurse who is treating is very precise with their treatment then the delivery of energy is extremely accurate.