Code page identifier
From Just Solve the File Format Problem
A code page identifier is a 16-bit integer that identifies a character encoding. The numbering system was originally developed by IBM. The identifiers were intended to be globally unique, but due to lack of vendor cooperation, there are cases where the same encoding has been assigned different numbers by different vendors. (It's unclear whether there are also cases where the same number has been used for more than one encoding.)
Examples of such identifiers are 37 for IBM EBCDIC USA/Canada, 437 for CP437, 1252 for Windows 1252, and 28591 for (Microsoft) ISO 8859-1.
See Character encoding for information about specific encodings.