Tracking categories field refers to Location/Class codes in QBO, Tracking codes in Xero, Projects in KashFlow and so on. Sometimes referred to as dimension or account extension as well.
These can help to better organize records or collect custom expense data for your company.
Within the document recording process, you can choose to select a tracking that the document relates to.
You can also split the item between several projects or objects by splitting the document beforehand and using the corresponding tracking.