Each segment is normally one of the twenty amino acids used in Foldit. In some puzzles, one or more segments may represent a "ligand", an atom or small molecule which interacts with the protein. These ligand segments are not "residues".
Segments are numbered starting at 1.
To see more information about a segment, hover over the segment with the mouse and hit the tab key. This opens a "Segment Information" window showing the segment number, the amino acid for the segment, the segment's overall score, and the score parts that make up the segment's score. There may be additional fields shown for some puzzles.
In recipes, many Foldit Lua functions accept a segment number as an argument. This allows a recipe to retrieve information about a segment, use a tool on a segment, or change certain information about a segment.
Some Foldit puzzles contain DNA or RNA. Although the chemistry is different, DNA and RNA are polymer chains. DNA and RNA are chains of nucleotides or "bases". Each base in the chain is assigned a segment number. Many of the Foldit Lua functions work on DNA and RNA in much the same way they do on protein.