Cross over altitude is defined as the altitude at which the climb law of aircraft switches from Indicated Air Speed to Mach speed. Cross over altitude varies with cost index and plays vital role in climb economy calculations.
TAS is maximum at cross over altitude and starts decreasing thereafter up to tropopause and is constant from there on. Difference between cross-over altitude and optimum altitude helps in deciding cost effective climb strategies.
Generally for light/medium category aircrafts the difference is 6000-9000 feet (optimum altitude is higher than cross-over altitude). More emphasis is required on fuel economy in such case. For heavy category aircrafts this difference is minor. Advantage of TAS increase till cross over altitude can be taken for such flights and more emphasis is required to be given on time economy.