I am attempting to define CA obligations vs. agreed tasks. Advice/references?

For whom?

David Brooks, County and District Law, TX Practice Series is very good on duties of office. The state also has a guide at http://www.orca.state.tx.us/pdfs/7TLCOGDE.pdf. Some counties have specific legislation for their CAO that details duties not required of other counties. Some commissioners courts fund services not otherwise required by law or statute. I have always considered it a county by county look.