What is the difference between "onXLE" and "callback"?

When loading data into the grid, I can use callback function or “onXLE” or both.
What’s the difference between them?
I noticed that “onXLE” fires first and after that the callback. This is the order of execution everytime? This is the only difference?

There is no actual difference between the onXLE event and a callback function of the load() method.
The main difference, that sometimes you need a common code to be launched after the data loading, and in other situation you need a specific code to be called for after the specific data loading call.