S2S IPsec vpn || unable to understand ||

Hello,

They are just simplifying a bit how remote selectors and which interfaces will be added into firewall policies.

None - you will need to manually specify local interface, local and remote selectors

Share Wan - you will be asked for local wan interface. What it will do is that it will add this interface into firewall policy because it expect that the remote firewall will use this firewall as internet access also. More info:

https://docs.fortinet.com/document/fortigate/6.2.0/cookbook/301182/tunneled-internet-browsing

Force to use remote WAN - it will configure 0.0.0.0/0 as remote selectors and then install it as static route - it will be default route and traffic will be send to remote firewall.