  We analyse unstable D-brane systems in type I string theory. Generalizing the
proposal in hep-th/0108085, we give a physical interpretation for real
KK-theory and claim that the D-branes embedded in a product space X x Y which
are made from the unstable Dp-brane system wrapped on Y are classified by a
real KK-theory group KKO^{p-1}(X,Y). The field contents of the unstable D-brane
systems are systematically described by a hidden Clifford algebra structure.
  We also investigate the matrix theory based on non-BPS D-instantons and show
that the spectrum of D-branes in the theory is exactly what we expect in type I
string theory, including stable non-BPS D-branes with Z_2 charge. We explicitly
construct the D-brane solutions in the framework of BSFT and analyse the
physical property making use of the Clifford algebra.
