A statement has existential import when its truth depends on evidence for the existence of things in a certain category--in the case of categorical propositions, the existence of things in the categories signified by its subject and predicate terms.
Existential Import and Categorical Propositions
Consider the following propositions:
All unicorns have horns.
No perpetual motion machine has been patented.
Both are true even though there are no unicorns or perpetual motion machines. Thus, these statements lack existential import.
Many modern logicians hold that existential import is a function of a statement's logical form. According to this view, universal categorical statements in general do not have existential import.
Statements that are particular in nature, however, do have existential import. To say that some S are P, or that some S are not P, is to imply the existence of Ss; if there are no Ss, then both statements are false.