In his book, "Principles of Mathematics", Russell makes the following claim:
Being is that which belongs to every conceivable term, to every possible object, of thought-in short to everything that can possibly occur in any proposition, true or false, and to all such propositions themselves. Being belongs to whatever can be counted. If A be any term that can be counted as one, it is plain that A is something, and therefore that A is. 'A is not' must always be either false or meaningless. For if A were nothing, it could not be said not to be ; 'A is not' implies that there is a term A whose being is denied, and hence that A is. Thus unless 'A is not' be an empty sound, it must be false. Whatever A may be, it certainly is. Numbers, the Homeric gods, relations, chimeras and four-dimensional spaces all have being, for if they were not entities of a kind, we could make no propositions about them. Thus being is a general attribute of everything, and to mention anything is to show that it is.
Existence, on the contrary, is the prerogative of some only amongst beings. To exist is to have a specific relation to existence-a relation, by the way, which existence itself does not have." (Pages 449-450).
Although Russell makes this dinstinction between existence and being, I have been unable to find any further comment on the details of this distinction - I would be interested to know what he thought.