  This article is part of a project which consists of investigating Arthur
packets for real classical groups. Our goal is to give an explicit description
of these packets and to establish the multiplicity one property (which is known
to hold for $p$-adic and complex groups). The main result in this paper is a
construction of packets from unipotent packets on $c$-Levi factors using
cohomological induction. An important tool used in the argument is a statement
of commutativity between cohomological induction and spectral endoscopic
transfer.
