Both relate to flow control and trying to deliver a lossless network medium. Regarding B2B credits / Fibre Channel and the hop-by-hop windows in X.25, the idea

Both relate to flow control and trying to deliver a lossless network medium. Regarding B2B credits / Fibre Channel and the hop-by-hop windows in X.25, the idea is that each hop has a counter of how many packets it will send to the next hop without receiving an acknowledgement, which are "credits". Once you're out of credits, that hop stops sending traffic. X.25 was superceded/replaced by TCP/IP (which uses end-to-end acknowledgements, instead of hop-by-hop) due to increased performance. But B2B is used with Fibre Channel (storage) networks, which require lossless performance on each hop. Regarding PAUSE frames and Ctrl-S/Ctrl-Q, the idea is that as a device's receive buffer fills up (it is receiving data faster than it can remove it from the buffer), it will hit its 'pause threshold', and it will send a PAUSE message to the sender to let it know that it can't handle any more data. This is similar to Ctrl-S/Ctrl-Q, which are XOFF/XON control commands in flow control, to stop and start traffic as you ran out of buffer space.