This is the % of maximum humidity (at water saturation) of a gas. The upper limit of water removal with a given amount of dryer gas is to saturate it to 100% RH.  At the beginning of the dryer, liquid water is available on the chip surface, or readily available from the pores, and removal is fast, but the rate drops off considerably as the wood dries and diffusion of water vapor from within the chips becomes limiting. This pattern allows very hot dryer gas (up to 500F) to be used at the front end of the dryer without overheating the chips, but dictates much lower temperatures (200F) at the back end of the dryer to prevent “blue haze”.