Let’s start with what is code stamped vessel

It’s a confirmation by manufacturer that all code requirements are complied (including few non-critical/documentary ones like following a particular edition of code, format and font size on name-plate, format of data report etc.)

Note that it is a confirmation by manufacturer, though they have to appoint an  Authorized Inspector (AI) to check and validate the claim. However, final responsibility lies with manufacturer only.

Now for a non-code stamped vessel, we have two options

The manufacturer can still comply to all code requirements at his choice or as part of commercial agreement with user. The only benefit here will be the cost of AI involvement.

The manufacturer may take few deviation from ASME code as they don’t have to certify compliance. This should be in consultation with user. For example, the user may allow the manufacturer to follow a previous edition of code. In any case though, such deviation should be carefully reviewed and limited to not compromise the overall safety of equipment.