Magento: ?___SID=U appearing in some urls

Question

Can someone please explain why ?___SID=U is appearing in some Magento URLs on my site and not others?

I think it has something to do with sessions but I am not entir

Answer 1

The SID is a "session ID". Magento uses this to track a user's activity within the same Magento installation. Normally, Magento powers one website and one store from one installation (database).

Magento could power multiple websites with multiple stores from one installation though. The SID allows users to stay logged in while navigating across these websites/stores.

I think if you have the function enabled, the SID is sent when accessing catalog URLs so Magento can update the session with the user's location/state for the current website/store.

If you're not running a multi-website or multi-store environment, it's safe to disable the SID on the frontend.