IYO, which standard is the least restrictive in designing a tank?

Appreciate any enlightenment...

Found what I was looking for and thought I would share,

API 650 covers flat bottom tanks, and 620 also covers flat as well as other shapes. 620 max design temp is 50 degrees F higher than the 200 degrees F of 650. 620 covers up to 15 psi internal and 650 up to 2.5 psi (appendix F becomes applicable in case of internal pressure). 650 is vertical tanks only, 620 is vert and horizontal.