Channel in global memory

It's based on a FIFO so if the FIFO is shallow enough it probably gets synthesized with registers. It doesn't have the ability to change to offchip memory but you could implement an offchip FIFO, put it into the BSP and then communicate with it using channels. You would still end up with some channel FIFOs but they would connect to a much deeper FIFO that uses offchip memory for storage.

Of course you could also just model this with a kernel that has an input and output channel that buffers to global memory. Then your other kernels move data in and out of this "FIFO kernel". Then you don't have to write RTL or mess with the BSP, it's all just C at that point.