Cf. is an abbreviation for the Latin word confer, meaning "compare." Cf. is a signal indicating that the cited source supports a different claim (proposition) than the one just made, that it is worthwhile to compare the two claims and assess the difference. Parenthetical explanations are encouraged to explain how a cf. source relates to the text.
In other words, a cf. source simply offers a different yet non-contradictory claim and actual support to the claim just made should not be assumed. If the source gives a contradictory claim, a negative signal should be used.
[Last updated in July of 2022 by the Wex Definitions Team]