Europe 863-870 MHz (SF9 for RX2 - recommend)

there has been a discussion about the frequency plan HERE, but I´m still not clear about the effect.

Can somebody explain (in simple words) what the difference ist for a sensor node to choose one or the other plan?

For a node you should choose the TTN recommended SF9 for RX2…

The difference is the spreading factor used for downlink packets when a gateway is instructed to send them in the second downlink window. If you choose SF12 the range will be larger, however the gateway takes 8 times as long to transmit that packet (and during transmission a gateway does not receive so it potentially misses more uplinks). Because the gateway is subject to airtime limits like the nodes it also decreases the amount of downlinks a gateway can potentially transmit by a factor 8.
To mitigate the range reduction from SF12 to SF9 the gateway does transmit with larger power (which is allowed at that frequency) so effectively the node should be able to receive the downlink.

Thank you much for the explanation. That was really not clear and I did not find anything in the documentation.