Glad to know you've found it helpful.
I'm learning too so probably I'm not the right person to ask for detailed explanations, however , those lines are used to manage a connection coming from WAN to be answered out on the same WAN
First the incoming connection from a specific WAN with no marks is marked as WANx->LANs
Then the packets flowing out from LAN (address list=LAN) , with WANx->LANs mark are further "routing-marked" as ISPx_Route to be associated to relative ISP route into the IP routing table: look at previous page with :
add gateway=188.8.131.52 routing-mark=ISP1_Route distance=1
I hope this is clear (and not a mistake.....)