A more thorough explanation:

Negative signal

A negative signal is a type of signal that indicates a contradiction between a stated proposition and the source about to be cited. It is used to admit that contrary arguments exist than whatever is being presented here. Commonly used negative signals are as follows, in order from mutually exclusive to somewhat contradictory: Contra, But see, and But cf.

Example 1: Contra

John believes that eating meat is healthy for humans. Contra, a study published in the Journal of the American Medical Association found that a vegetarian diet can reduce the risk of heart disease.

Example 2: But see

Many people believe that climate change is caused by human activity. But see, a recent report by the Intergovernmental Panel on Climate Change suggests that natural factors may also play a role.

Example 3: But cf

Some studies suggest that drinking red wine can be good for your heart. But cf, other studies have found that even moderate alcohol consumption can increase the risk of certain cancers.

These examples illustrate how negative signals are used to acknowledge the existence of contradictory arguments or evidence. By using negative signals, the writer or speaker is able to present a more balanced and nuanced perspective on a topic.