It felt to me Callback is same as partial by functools,
So I was intrigued to exploit it more to understand it better.
Callbacks are a general concept in programming where, as an argument to a function/method, you pass a function to be called, usually one which will often be called upon every occurrance of a specific event in the ui or system.
PyMEL provides some simplified access to the API that can be
used to associate your own functions (callbacks) to the events that occur in Maya.
This command allows you to add callbacks at key times during UI creation so that the Maya UI can be extended.
Functools (the 'partial' you mention) is a set of tools to make it easier to pass arguments into functions that are being used as callbacks.
The return from a new partial object which when called will behave like func called with the positional arguments args and keyword arguments keywords. If more arguments are supplied to the call, they are appended to args. If additional keyword arguments are supplied, they extend and override keywords.
- Joe Crawford