The difference between 'x' and 'z' is that 'z' is a known state of high impedance, meaning actually disconnected The pin is able to read input high vs input low signals on the pin As such, it could be driven to any other value with some other driver.
Crazy Shit-PSD. by OneDirectionIsMyLife on DeviantArt
Usually the output impedance of a digital circuit is quite low so that it can set the line to which is connected in high or low logic state, that means high (close to vcc or vdd) or low voltage (close to ground).
