frequency accuracy vs frequency stability
In many cases today, the crystal frequency reference +/- ppm drift is the same as the transmitter frequency drift in ppm.
In older designs, such as FSK systems where the modulation is AC summed with the PLL control signal, then for certain data streams you can have an apparent frequency walk off of the desired frequency. This is primarily a problem if there is no randomizer used in the digital data, or if there are long periods of the same bit transmitted.