Steve is correct.
ASME B&PV Code is for 15 PSI and up. Below 15 PSI, there is usually not a specific legal requirement for one standard or another. But normally, API-620 would be used for pressures between 2.5 and 15 PSI, and API-650 for pressures below 2.5 PSI. AWWA D100 and NFPA-22 are other atmospheric tank codes. There are others for UL tanks, bolted or concrete tanks, etc.
For a tank to hold 15 PSI, it has to be designed for that pressure. You can't just take any old tank, and put 15 PSI into it. And designing a tank of some size for 15 PSI will usually require a dome roof or steep cone roof, anchor bolts, etc.
Eabrar, as to your question- a tank is always cheaper than a pressure vessel of the same capacity. So for any product that can be stored in an atmospheric tank, that's the way to go. And when the nature of the product, or the process conditions require pressurized storage, then you go with the pressure vessel and pay the price.