Let's say you want to track items in transit, meaning you are transferring some items from main location (source) to west location (destination).
Once you start a transfer order, you ship items from main location to one of "in transit locations". If for example you own trucks that will transport your items you may use OWN LOG (your own logistics to move the good). Since transport can last for some time such items will then be placed in OWN LOG location, in other words they will be in transit.
Now when they arrive to destination location, you will receive such items from OWN LOG location to your destination location using transfer order.
You can read more about this here:
https://docs.microsoft.com/en-us/dynamics365/business-central/inventory-how-transfer-between-locations
