MetaKey it is an integer but it is used as a bitfield. MetaKeys are "none"(0), "shift"(1), "control"(2), "alt"(4) and "META"(8) (windows key).
This MetaKeys can be combinded for example combining control and alt: 2 + 4 = 6. When the user holds control and alt and presses a oskey the event will only run when it was registered with metakey 6.
Inside the press event MetaKey-Keys always have the metakey they start themself or a bigger one if other MetaKeys are hold down during that process.
When holding down all Metakeys the result would be 1 + 2 + 4 + 8 = 15