What is the difference between high-Z (high impedance ) and open circuit?
If an impedance is referred to as 'high-Z', it usually means it's high enough to be approximated as an open circuit, in the context of other impedances in the system.
For instance, my 1 MΩ // 30 pF oscilloscope input is high-Z if I'm measuring audio in 600 Ω, but not if I'm measuring signals at the other end of a MΩ resistor, or trying to measure RF (the 30p pF is too low impedance at RF)