Process Engineers would say Internal design pressure 0 psia, external pressure assuming above ground at sea level 15 psia.
Why?
Sometimes you can have e.g a vessel full of steam which then cools and condenses to water at a very small fraction of the volume.
Sometimes you can connect your vessel to a pump or compressor which pulls all the contents out of the vessel.
Many times it won't be absolute vacuum, but FV is better than trying to guess what level of sub atmospheric pressure you can rely on.
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