In my case it was as follows: My Fritzbox has a 2.5GBe socket. From this output I went to one of the 2.5GBe sockets on the Qnap. The Mac has a 10GBe output. So I go to one of the two 10GBE sockets on the Qnap. From the second 10GBe socket I then went to the 10GBe from the Synology NAS. I had a stable 950-1020 MB/sec. from the Mac to the NAS. And the full 1000Mbit from Vodafone for internet.
You can also connect 2 devices with 10G, e.g. PC and NAS and other devices like routers with 1 or 2.5G. Makes sense with a 1G internet connection but fast local devices.
Yes, that is correct. This switch is intended to connect one device via 10 GBit and other devices via 2.5 Gbit. 4x 2.5 also results in 10, so that makes sense... 😉