74HC4052 Multiplexer Help

I'm looking at the datasheet for this and I can't figure out what the nZ input/outputs are for. Can anyone explain?

https://www.qsl.net/l/lu7did//docs/QRPp/74HC4052.pdf

It has two 4-channel switches. The switches couple signals forward or backward.
Each of the two switches has a common in/out that they call 1Z and 2Z. "n" is the number of the switch (1 or 2).