That's a broad question with little detail to provide an answer, but here's some info to help get you started.
Raw components may be purchased to specifications, such as provided by ASTM.
For example, a specific ASTM grade of carbon steel shape certification is:

ASTM A992/A992M-11(2015) Standard Specification for Structural Steel Shapes

This is only one example of many such specs...

However, the only place to find the real answer to your question, whether for material purchase or fabrication specifications, is in the contract documents that govern the tank construction.

Tim