Normally, the basis of an AOMX calculation consists of all possible Slater determinants for a given number of d electrons. Sometimes, howewer, it may be desirable to make the basis smaller, for instance in order to speed up the calculation, to suppress uninteresting output or for the purpose of studying CI effects.
With the BAS directive you can influence the basis set size in two different ways: by giving a minimal spin multiplicity and by giving minimal and maximal orbital occupations.