"Chassis" what is the meaning?

[noisepic]

In some PCB we have chassis, what is the purpose of "chassis" in PCB. in which case we connect to GND, or let NC, or connect to VCC or other power supply.
​Could you clear it for me?

[robertferanec]

PCB Chassis ground may be connected to chassis of the product or system. For example if your board is mounted inside a metal box, the metal box would be called chassis and you connect chassis ground of your PCB to the metal box of your product. Sometimes you can find also name enclosure. I am not native English, so I am not 100% sure if there are some differences between chassis / enclosure, but they usually mean it is the box where your PCB is mounted in.

For some measurements, tests, systems, situations you may design your system the way, that you separate board GND from chassis GND and then you may find all the kind of different connections between these grounds (e.g. they may be directly connected, not connected, they may be connected by a high voltage capacitor, by BEAD, ....) - the final connection will depend you specific application.