A more thorough explanation:

In formal legal writing, "see also" is a signal that indicates the cited source provides additional support for the statement after primary supporting sources have already been cited. This means that "see also" citations are used to follow sources that have been cited with the "see" signal or no signal at all.

For example, in a legal brief, a lawyer might write:

The defendant's actions were clearly negligent. See Smith v. Jones, 123 F.3d 456 (2d Cir. 1997); see also Johnson v. Smith, 456 F.2d 789 (2d Cir. 1972).

In this example, the lawyer cites two cases to support the argument that the defendant was negligent. The first case is cited with the "see" signal, indicating that it is the primary source of support. The second case is cited with "see also," indicating that it provides additional support for the argument.

In general writing, "see also" may be used not as a signal for supporting sources, but as a way to suggest other ideas that may be of interest to the reader because of their similarity to the discussion at hand.

For example, in an article about climate change, the author might write:

The effects of climate change are already being felt around the world. See also the recent report by the Intergovernmental Panel on Climate Change, which warns of dire consequences if action is not taken soon.

In this example, the author uses "see also" to suggest that the reader should look at the report by the Intergovernmental Panel on Climate Change for more information about the effects of climate change.