We're glad you asked!
In our Track and Trace API, we offer the options to either
subscribe to an AWB and automatically receive a call back every time we detect a change in milestone
OR call the APIs every time you want to retrieve the latest status update on an AWB.
In a nutshell the first mechanism PUSH-es you the updates whenever they are available and the second one requires you to PULL (call or query) the API for a status update.
The 2 options fit different workflows and also largely depend on your estimation of how many status updates you'll need (on average) per MAWB, due to the different pricing structure.
Any question, just let us know!