All reference signals are endogenous, generated within the organism. Powers followed Ashby’s assumption that some corporeal states essential to survival are intrinsic, hence, intrinsic variables and their reference values. The usual suggestive examples include core temperature and blood oxygen level.
There follows also intrinsic error as a cause of reorganization. It may be that persistent error to other controlled variables can also initiate reorganization.
Because of evidence that changes in synapsing and brachiation are the default behavior of nerve cells, it may be more accurate to say that intrinsic error (& perhaps also persistent error) disinhibits reorganization. Perhaps systems that are controlling successfully maintain a local neurochemical environment that inhibits random changes to their structure.