Question 1: Yes the checkbox (flag) is needed. This indicates to CAESAR II that there is allowable stress data defined on that element.

Question 2: Yes. Allowable stress data is duplicated (carried) forward until changed. A change to the allowable stress data will require checking the "allowable" checkbox.

Question 3: Yes, any time you change a temperature, or a material, or the piping code the software should check the "allowable stress" checkbox.