What's the purpose of 'Subscription Renewal' tab?

The Subscriptions view has selection/filter tabs for Plans, Subscription type and Subscription Status. This is useful for showing (and downloading to .csv), for example, all expired subscriptions or just those subscribers to a particular plan. However, the selection tab labelled 'All Subscriptions', which has a drop-down list of: 'New Subscriptions', 'Subscription Renewal' and 'Subscription Upgrade' doesn't seem to do anything. Our site has many members who renew offline, requiring manual renewal at the back end. I want to see and download a list of just those who have been renewed in this session and would expect that to be enabled by selecting 'Subscription Renewal' from the drop down but clearly this isn't going to happen. So what's the function of this tab?

As far as I know, the subscription renewal filter shows those subscriptions that have been renewed vs. the new ones. You would have to have a renewal menu item that your subscribers would use when they renew to see this happen. If your subscribers are re-subscribing by creating a new subscription each time, you wouldn't see anything in this filter.
Hope this helps.

So, are you saying, Brian, that this only works if a plan is set to 'Recurring Subscription = Yes'? All our plans are not recurring.

No, our plans are not recurring either, but we have a separate menu item to renew an existing subscription. This I believe is what the filter will show, those that are renewed vs. a totally new subscription.

Disregard my other response, Brian. If by the 'separate menu item to renew an existing subscription' you mean the 'Renew Subscription' button on the Subscriptions page (back end) then that's what we are doing already. The renewal works by creating a new subscription record (not Subscriber record) for the subscriber. By definition, if a plan is not recurring a new subscription record must be created. None of our plans is recurring so all renewals should create a new subscription record. So far so good but this is where the 'Subscription Renewal' filter on the subscriptions view doesn't work (i.e. nothing shows). I can't see any other way in which we could handle renewals manually. As it happens, we also have a separate menu item (front end) for members to renew but as far as I can tell this performs exactly the same function as the backend 'Renew Subscription' button with exactly the same result, i.e. nothing to see if the 'Subscription Renewal' filter is applied.