Assuming onsave is required, the following 2 results were found.
to: Joomla! CMS 2.5.x Joomla Filter: Validation OnSave INT Forces the value to be an Integer UINT Forces the value to be an absolute number (-4.2 becomes 4.2) FLOAT Forces the value to be a floating point value BOOL Forces the value to be true or false...
which is a lookup to the "Skills" module. This client script is triggered when the user hits the "Save" button (event: onSave) and will stop the user from saving the record and continuing if 3 different skills are not specified in the subform: //...