PK (Place Keeper) - You need to know where you are in doing something, keep track of things, especially if you need to set something aside to do other things before you can go back to something and continue.

In simple terms:

The PK command manipulates the PK file contents, as well as creates, saves and changes/loads which PK file is being used. The contents of the PK file is relative to the VIC commands of the VIC instance the PK file is associated to. By manipulating the contents of the PK file you control the VIC instance in regards to where you are, what you are doing, where you are getting input from, where you are sending output to, etc..