I've been doing a little bit of reading on string theory, and the very large number of string vacua that are possible (i.e., perhaps 10^500 or more). One thing that is not clear to me is exactly what constitutes a 'vacuum' in string theory. In QFT theory, the vacuum is defined as the state with no physical particles (i.e., the ground state)... however I'm not sure, and I haven't been able to clearly find a source, which says that the same is true for string vacua? I do understand that the choice of string vacuum ultimately determines what particles may exist, etc. I would surmise that to call anything a 'vacuum,' it would have to be devoid of particles (excitations above the ground state), but not 'empty' (e.g., filled with fields, etc., just as in QFT). Is this true for string vacua?