not a lawyer, but one of those dumb lawyer boiler plates that describes what they need to do to make the service work, distribute and display your content, but written in such a way that it also covers selling your shit on tshirts

Also the “transferable” part seems weird to me, so they can give it to whoever they choose should they want to

I’m being generous, but I think that is to cover that the data could end up on other servers that don’t belong to them to be served. The sub license allows them to transfer their rights to copy and serve to those partners