You should mail legible copies of all your Form DS-2019, Certificate of Eligibility, along with the written request for an advisory opinion. USCIS does not recommend sending your request by fax because forms frequently become illegible during the fax process.
The skills list is a list of occupations that are needed in the exchange visitor's home country.
The case number is at the top portion of the letter you will receive after you have submitted your Data Sheet application and the processing fee. The letter will explain the procedures and documentation necessary for your waiver review request. If there were prior applications, the case number remains the same no matter how long the case history becomes.