A signal indicating that the cited source supports something similar to the opposite of whatever the author just said. In other words, the cited source probably conflicts with the author's claim. But cf. is the weakest of the major negative signals.
Note that if a but cf. citation follows another cited source, the but cf. source will be construed as conflicting with the previous source - and not the statements in the present piece of writing. Therefore, to prevent confusion, a but cf. citation should be converted into a cf. citation if it is placed after another source with a negative signal. For more on this rule, see order of signals.
See Negative signal.