Let's try it by way of example. And let's assume first that you had a property where your images were set to be Per Session caching.

So the first time I visit your property I have none of the images cached. Meaning the first time I visit my RLC client is going to go out to your web server to fetch all of those images for me.

Then I leave your property for a bit, and come back 30 minutes later. In that 30 minutes I have not logged out. Just flitted around in world to a couple of places.

Because I'm still in the same Session (I haven't logged out, remember) those files should all get loaded from the local cache on my PC. As opposed to the RLC client having to go out to fetch them again from your web server.

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On the other side of that coin, if I'd visited your property, but then logged out of RLC before visiting your property a second time, this would constitute a New Session. So in that case my RLC client would need to go fetch the images from your web server again.

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And Forever overcomes this logging in and logging out stuff. With Forever, in theory at least, I should never have to go to your web server to fetch an image after my initial visit. Whether I log out and log in, or whether days or weeks have passed, it should make no difference. every subsequent visit should pull the images from the local cache on my PC.

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Now, as to which is better...

When I'm building a place I tend to go ahead and leave it at the default Per Session. Mainly so that I can see changes as I make them. Often times I'll need to change a texture to Never just so I can get it up update for me. But this is only as I'm building things.

Once I have the images looking the way I want them to, I set everything that's not going to change often to Forever. That way nobody who visits has to re-download all of those images each time they visit the property, speeding up their region load time a bit.

Great Information! Explained simple.

I understand that the cache contains the textures etc, but I know nothing of the file structure.

Does each prop with a texture attached have it's own texture in the cache?
Or is there only one copy of each texture in the cache and every prop with that texture loads the same cached file?

Thank you

Just a additional note to above, every prop using same texture has to be set to forever also or by default it will upload from the web.