Well, PNID (primary network identifier) is a VMware internal term and it is officially called "system name".
But my question to the questioner was why he needs to know something about PNID. I have got the expected answer. The questioner did some research how to change vCenter IP address and hostname.
So let's discuss these two requests independently.
First thing first, vCenter hostname cannot be changed. At least for vCenter 6.0 and 6.5. It may or may not change in the future.
On the other hand, vCenter IP can be changed. However, system name (aka PNID) is very important when you are trying to change vCenter IP address. vCenter IP address can be changed only when you have entered FQDN during vCenter installation. In such case, PNID is the hostname. In case, you did not enter FQDN during vCenter installation, the IP address is used as PNID which would end up in the inability to change the vCenter IP Address.
Below is the command how to check in VCSA what is your vCenter PNID.
In this case above, PNID is the hostname (vc01.home.uw.cz) so I would be able to change IP address.