The symbol a is called an argument of M . The predicate M may be applied to any individual, and that individual is then an argument of M . In general, if x is any individual whatsoever, then Mx is the assertion that x is a man. This assertion may or may not be true, depending on what x is. The expression Mx is called an atomic formula of the predicate calculus.
Some predicates require more than one argument.
Thus B requires two arguments, and Bxy is an atomic formula. This is not an atomic formula or any other kind of assertion. It is only a meaningless combination of symbols. In analogy with English grammar, we could say that Bxy is like a grammatically correct sentence, while Bx is merely a sentence fragment. Such fragments play no role in the predicate calculus.