I am working on a custom Magento module. I am facing some issues to debug the code and one question that comes into my mind is that what is the difference between offline and online shipping method. Try to elaborate Magneto docs explain Offline Shipping here but I have to understand it more deeply
The online shipping methods are those which actively consume the API of a 3rd party system owned by a shipping services provider in order to fetch the shipping cost and the information regarding the parcel's delivery tracking in Magento.
So I would say that if you are not intending to integrate Magento with the platform of a shipping carrier, then you can develop the module as an offline shipping method by implementing and keeping all the logic inside Magento.
There are situations where you already have the shipping price logic in Magento, but the ERP/ or another system which handles the orders will integrate with the shipping carrier system and receive the tracking number of the order. This, in my opinion, is also offline shipping method scenario, even if there is a way for Magento to receive back the tracking number from the ERP so that the client can track their orders.