Sharepoint: User Type field in schema.xml

I was writting schema.xml for Custom List Template, there i need field of type User like below

I was setting value of ShowField property to Job Title which i was using in CAML query to filter out the users. I searched lot for ShowField values of User type but i coudn't found anything from MSDN, even not an expected results from google. I am very much habitual of seeing the View Source of the HTML pages so i see the page source of Task list's User column and i got all the values from that source. I think looking out HTML pages source is not a bad practice :)