When should Upstream (WAN Interfaces) be manually selected?

[fbeye]

So after reading some information on Upstream gateways, I still have no idea if and when to utilize the option.
In tearing down the system I have, when it was simply LAN/WAN I never needed to manually set the WAN Interface as an upstream.
Now I am running 2 OpenVPN Clients (with their designated LAN IP Ranges, their appreciate LAN Rules and their Outbound NAT rules) and am curious if with this all set do I need any of them, or all (vpn 1 vpn 2 or wan), to be set as Upstream?
I do have LAN rules for the ip ranges to "out" on the correct VPN Interfaces. I just wanna have the correct settings even though if let's say either way would work. I want the more secure and legitimate setting.

[fbeye]

Any possible suggestions? Like I am reading what Upstream means but I just can not integrate it to a solution for my scenario that makes sense to me.
When [Interfaces] WAN, VPN1 and VPN2 are active do I need to "enable" Upstream on all 3 Interfaces or is this done via Outbound NAT/ LAN Rules [out]?

[fbeye]

It seems when ONLY WAN [PPPoE] is enabled, Upstream does not need to be selected but if I have WAN/VPN1/VPN2 enabled, I have to have VNP1 and VPN2 Upstream enabled.
I assume this is the way it is supposed to be.

[mimugmail]

Upstream Gateway in Interface section Sets a rule that every packet coming in on this interface gets replied to this gateway, also if source was on the same LAN. Also Auto nat rules get generated with interfaces with upstream. For pppoe and dhcp this works automatically

[fbeye]

Well that explains why when WAN/PPPoE only, I need not set up Upstream, but when using VPN1/VPN1 I need to enable each Upstream for the reason you mentioned. Thank you.