What does online transfer ref mean?

Excuse me, could you please clarify what 'online transfer ref' stands for in the context of cryptocurrency and finance? I've come across this term a few times, but I'm not entirely sure of its meaning. Is it related to a specific type of transaction or a reference number used for tracking purposes? It would be greatly appreciated if you could provide a concise explanation of what this term means and how it's used in the world of digital currencies and financial transactions.