Specify the [Object] data type to allow users to select another object as the value of the attribute. You must specify an Object type (metaclass) to link to, and can optionally specify an Object stereotype to filter the objects available for selection and an Inverse collection name, which will be displayed on the Dependencies tab on the referenced object property sheet.
For example, under the Table metaclass, I create an extended attribute called Owner, select [Object] in the Data type field, and User in the Object type field. I name the inverse collection Tables owned. When I set the Owner property of a table, the table will be listed on the Dependencies tab of the user property sheet, under the inverse collection name of Tables owned.