Man Machine Interface. You may get additional information from tracking actual user behavior. On a web site you can simply use the server logs.
Several programs are available for analyzing server logs.
On other systems you may set up a system for logging everything that users do. These logfiles can then be analyzed statistically. If the statistics reveal that many users are making the same errors or have the same problems, then you have found a usability problem that should be solved. The statistics may also be useful for dividing users into categories. If those users that use function A also use function B, while other users use only function C and D, then you can use this information in the design of your menu system so that A and B are in the same submenu, while C and D are in another submenu.