There are values you add in in case there is no CRYST1 entry right?

It just defines the value for this parameter from the documentation. It's
in the first note in the link Morten provided.


> If the structure was not determined by crystallographic means, simply
> defines a unit cube
> (a = b =c = 1.0, alpha = beta = gamma = 90
> degrees, *space group = P 1*, and Z = 1)


As for an explanation, P1 means no symmetry, so it's appropriately used for
structures where there is no crystallographic unit cell.