This tab is displayed only when one of the bulk keys described here is used or when the following key is added to the AppSettings section of your web.config file:
<add key="CMSDebugWebFarm" value="true" />
On the Debug -> Web farm tab, you can see which web farm tasks were logged and which servers were notified about the changes. This is particularly useful if you want to find out if the web farm works effectively and synchronizes correct data, or generally if you want to troubleshoot web-farm-related issues.
The bottom part show the logged tasks, while the top part shows asynchronous notifications of other web farm servers. Tasks with the DO: prefix are synchronization tasks received from other servers which were (are being) processed.
The log can be cleared using the Clear web farm log button.