This is the full number of the residue, as used in PDB, comprising an integer number (positive or negative) and an optional single-character insertion code, generally a letter from A to Z. (Not available for alignment sequences.)
This is a string property (not an integer). Thus, both '1' and '1A' are valid residue numbers, but a Python integer (without quotes) is not.
To access just the residue number or the insertion code, see Residue.intnum or Residue.inscode.Example: See Residue.index command.