Forward and backward citation searching may be used to identify additional studies for your research topic that have not been located using scholarly databases. Both are reported in the PRISMA 2020 flow diagram as "records identified through other sources."
Backward citation searching is using the cited references from a key article. This is especially important to read the actual study for yourself or if you find several citations to the same study. Many databases provide references with links or you may search for each article's title in the databases.
Forward citation searching is used to identify more recent research citing a key article from your database search. Many databases provide "cited by" references with links. Or you can search for your key article's title in Google Scholar for the "cited by" option.
Any studies identified by citation searching are reported in the PRISMA flow diagram. Identified records follow the rest of the process for removing duplicates, title/abstract screening for eligibility, and using the full text to confirm inclusion in the final count of studies used for the review.