This is the number of the residue, as used in PDB. (Not available for alignment sequences.)
Note that the PDB residue number is stored as a 5-character string (not an integer). Conventionally, the first four characters are a sequence number, and an optional fifth character is the insertion code. For example, both '1' and '1A' are valid residue numbers, but a Python integer (without quotes) is not. Residue numbers longer than 5 characters will be truncated on assignment.Example: See Residue.index command.