It is known that the descriptor representation of linear smooth behaviors imposes no constraints on the partitioning of external variables into inputs and outputs. The issue is reconsidered here in the setting of impulsive behaviors. In this context there are two different descriptor representations that may be used: the "conventional" form and the "unconventional" one. It is shown that the conventional descriptor form again does not constrain the choice of inputs and outputs, but the unconventional form does. The latter representation only allows input/output partitionings in which inputs are "Dirac free". This notion is newly defined here. (C) 2001 Elsevier Science B.V. All rights reserved.