  A new possibility to explain the nature of thermal X-ray composites (TXCs),
i.e. a class of supernova remnants (SNRs) with a thermal X-ray centrally-filled
morphology within a radio shell, as a projection effect of the 2- or
3-dimensional shell-like SNR evolved in a nonuniform medium with scale-height
<10 pc is proposed. Both X-ray and radio morphologies, as well as the basic
theoretical features of this kind of SNR and the surrounding medium, are
considered. Theoretical properties of a shell-like SNR evolved at the edge of a
molecular cloud correspond to the observed properties of TXCs if the gradient
of the ambient density does not lie in the projection plane and the magnetic
field is nearly aligned with the line of sight. So, at least a part of objects
from the class may be interpreted within the framework of the considered
effect. The proposed model suggests that SNRs with barrel-like radio and
centrally-brightened thermal X-ray morphologies should exist. The model allows
us to consider TXCs as prospective sources of proton origin gamma-rays.
