The Debug -> IO tab is displayed only when IO debugging is enabled. This can be achieved either by configuring certain settings in Site Manager -> Settings -> System -> Debug, or by adding specific keys to the appSettings section of your project's web.config file. Both options are described in this context help topic.
On this tab, you can see which file operations were carried out on each request. For each operation, you can see its type, size of transferred data, number of accesses and the path to the accessed file. In the Context column, you can see the context from which the file access operation was called.
Enabling the Show complete context check-box displays complete context (not only the topmost item) in the Context column. The log can be cleared using the Clear log button.
For more information about debugging in Kentico CMS, please refer to Developer's Guide -> Development -> Debugging and system information.