But cf. is a signal indicating that the cited source supports something similar to the opposite of the claim (proposition) just made by the author. In other words, the cited source probably conflicts with the author's claim. But cf. is the weakest of the major negative signals.
But cf. serves as the inverse of cf. Just as cf. precedes a source that supports a claim (proposition) different from the original claim but analogous to it, but cf. is used before a source that supports a claim that is different from the contrary claim but analogous to that contrary claim.
See also: order of signals
[Last updated in July of 2022 by the Wex Definitions Team]