  A special class of quantum channels, named subspace local (SL), are defined
and investigated. The proposed definition of subspace locality of quantum
channels is an attempt to answer the question of what kind of restriction
should be put on a channel, if it is to act `locally' with respect to two
`locations', when these naturally correspond to a separation of the total
Hilbert space in an orthogonal sum of subspaces, rather than a tensor product
decomposition. It is shown that the set of SL channels decomposes into four
disjoint families of channels. Explicit expressions to generate all channels in
each family is presented. It is shown that one of these four families, the
local subspace preserving (LSP) channels, is precisely the intersection between
the set of subspace preserving channels and the SL channels. For a subclass of
the LSP channels, a special type of unitary representation using ancilla
systems is presented.
