If an entry contains multiple instances of the same field, they are all stored in the spreadsheet, separated by this value. If you want some other character(s) than the default, you can change it here. Just keep in mind that it should not be something you expect to occur in the actual contents of a field, or there will be conversion errors. For example a comma or a dash are probably bad choices here.
For the possible formats of separators, see here.
Created with the Personal Edition of HelpNDoc: Free iPhone documentation generator