Collection development policy today

What does collection development policy mean today, when the library has already some ancient and independent holdings ? We realize then that the word « collection » is not clearly defined. In French it has several meanings : private collection, publisher's series..., which increase the confusion, especially with collections considered as « sacred » entities. Therefore it is necessary to evaluate a library collection before accepting gifts from private collections and subscribing standing orders from publishers, whose principles are different from the library's.