Auxiliary Symbol Table records always follow, and apply to, some standard Symbol Table record. An auxiliary record can have any format that the tools are designed to recognize, but 18 bytes must be allocated for them so that Symbol Table is maintained as an array of regular size. Currently, Microsoft tools recognize auxiliary formats for the following kinds of records: function definitions, function begin and end symbols (.bf and .ef), weak externals, filenames, and section definitions.
The traditional COFF design also includes auxiliary-record formats for arrays and structures. Microsoft tools do not use these, instead placing that symbolic information in CodeView format in the debug sections.