I would really appreciate some help understanding the difference of these options.
Forgetting ALL about WAN connections for a moment, assume I have the following.
Router-1
LAN1 = 192.168.1.1/24
There is a PC attached to this LAN
Router-2
LAN-1 = 192.168.2.1/24
There are headless devices attached to this LAN
I want to route between the two devices so that the PC on LAN1 can access the devices on Router-2's LAN1
Logically then, I want to set up thus:
Router-1
LAN2 = 192.168.3.1
Router-2
LAN2 = 192.168.3.2
set a static route up that like this on Router-1:
Dest IP = 192.168.2.0
Sub mask = 255.255.255.0
Gateway IP Address = 192.168.3.2
Network Interface = LAN2
and a similar rule on Router-2.
To me, this is "For Routing Usage", so that's the option I selected, but it doesn't work, a trace from Router-1 stops at the LAN2 interface of Router-2 and doesn't hit Router-2's LAN1.
If I choose the "For NAT Usage" option, it works.
Please be gentle with me - why is this not a "For Routing Usage" scenario - or have I missed something ?
Thanks
Simon

Couldn't LAN2 on Router 1 be 192.168.2.0/24 - with the Inter-Lan routing set to route between LAN1 and LAN2.

LAN2 would then be physically connected to a LAN1 port on Router 2. Router 2 would only need a single LAN. Presumably a static route on Router 2 would be needed to find 192.168.1.0/24

I'm not sure which Router you're using - but the 'NAT or Routing Usage' is all WAN stuff isn't it?

I don't understand this at all... where is this 3rd 192.168.3.0 subnet coming from?

What's this got to do with "For NAT Usage" - which menu are you reading that from ?

Are these two routers side by side, or remote from each other ???

Sorry,I clearly haven't explained the setup well enough, so here's another try:
Router 1 - 2920-1
WAN - Disregarding the WAN completely, it's not in use here
LAN 1
LAN >> General Setup, active, DHCP enabled, IP address 192.168.1.1
LAN 2
LAN >> General Setup, active, no DHCP, IP address 192.168.3.1
Physical connections
LAN port 1 -> switch -> devices
LAN port 2 -> cable direct to LAN port 2 on Router 2
LAN >> VLAN Configuration
VLAN0 = LAN P1, P2 & P3 = Subnet LAN 1
VLAN1 = LAN P4 = Subnet LAN 2
no tagging enabled

Router 2 - 2920-2
WAN - Disregarding the WAN completely, it's not in use here
LAN 1
LAN >> General Setup, active, DHCP enabled, IP address 192.168.2.1
LAN 2
LAN >> General Setup, active, no DHCP, IP address 192.168.3.2
Physical connections
LAN port 1 -> switch -> devices
LAN port 2 -> cable direct to LAN port 2 on Router 1
LAN >> VLAN Configuration
VLAN0 = LAN P1, P2 & P3 = Subnet LAN 1
VLAN1 = LAN P4 = Subnet LAN 2
no tagging enabled

So, I want to route from devices on LAN 1 of 2920-1 to devices on LAN 1 of 2920-2, and vice versa.

The "For NAT Usage" option is available from LAN >> General Setup >> LAN 2 >> Details page, it's available for ALL LANs apart from LAN 1

The routers are "side by side" in a comms rack.

This is just an exercise in how to route the LANs independently of WAN connection(s)
Cheers
Simon

Can't you just use ONE router to make your life simpler?

I'm not sure I've followed it still but partly because I didn't have time to read thoroughly... I don't think you'd wan't NAT involved anywhere though but a static route should be correct.. but I'm not sure static routes can be used to route to a LAN side network from another LAN side.

LAN1 is always the 'NATted' LAN - but that relates to WAN-LAN, not to be confused with your multiple LAN subnets, which are private subnet ranges, but no NAT.

Thanks for the reply, there is an ulterior motive to wanting to get this to work.
Switching to IPv6 with two ISPs, so using a router on each ISP. As I want to use SLAAC for IPv6 addresses, each of the ISPs will have a different prefix - the BT prefix being dynamic, the other being an HE tunnel, so a "stable" prefix.
Using the IP4 "inter-LAN" routing would make it easier for me to get on to EITHER router (specifically from the Virgin/HE provisioned one to the BT provisioned one)
As I say, it's really an exercise as my current workaround is to use a VPN connection, just seems silly to go out to come back if you know what I mean.
I'm trying to avoid having any management from WAN options on.
Your comment about the "static routes ... used to route to a LAN side network from another LAN side" is really what I'm trying to get my head around, it seems like it can't be done like that.
Thanks for taking the time to have a look - much appreciated.
Cheers
SImon