Subpanel wires in conduit??

In a post above, it was said that wires going from a panel to a subpanel had to be either cable or in conduit?
Does that apply if the wires are behind paneled walls (both boxes are recessed) the whole way? If so, a specific code reference would be greatly appreciated.
Mine are connected with 4 wires; no cable, no conduit. I can't see why that is unsafe, as they are well protected by the walls; but want to comply with code. Thanks.
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Wade Lippman wrote:

If they were protected by a "loom" covering, that method would have been allowed as concealed knob and tube work up until 15? years ago. In any event, no longer recognized as one of the wiring methods. You would have to read the entire Article 300 to see which wiring methods are allowed. Jim
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had
greatly
that
with
The wires are insulated (ISE-2), but just not in cable. Sorry if that was unclear.
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