Throughout the paper let A be a semifinite, infinite von Neumann algebra acting on a Hilbert space H, .alpha. an infinite cardinal. The main purpose of our work is to give several characterizations of a class of closed ideals in A, by introducing the notions of relative ranks of elements in A and the relative .alpha.-topology on H. The relative .alphi.-topology is an analogue to the .alpha.-topology that we have defined in ([7], [8]). The present work is regarded as an extension of [7], [8] and motivated by works of M. Breuer ([1], [2]), V. Kaftal ([5], [6]) and M.G. Sonis [9].