The outputs are tested for voltage when sourcing or sinking 1mA output current. The graph tells you what is the typical voltage difference between the output and the supply rail (sourcing) or ground connection (sinking) when delivering a current between 0 and 7.5mA.
The inputs are tested for correct operation with 1uA being sourced into or sunk from the pin as an input.
You need to use the characteristics of the thing you are driving with your outputs before you know how much current the load will draw from them (up to the maximum of 7.5mA)
Looking through the data sheet, I can't see any specification of input capacitance, which presumably means it's negligible compared to the board tracking. We often take 10pF as a typical for 'a bit of track and an IC pin' if there's no capacitance spec. The current this takes to charge and discharge varies linearly with the frequency, so it's negligible for controls, and significant for a fast clock. If you're running (for instance) a 5v 10MHz clock into 10pF, then that's a charge of 5v * 10pF = 50pC each edge. So at 10M edges per second, that's a current of 10MHz * 50pC = 500uA.