And, to followup on Scott's suggestion, when you search on the strings
FILE SAS2XL
almost all the hits on support.sas.com have to do with using DDE and SAS to write data to Excel.
If you want to find out about formulas and TAGSETS.EXCELXP, then you would need to search on other strings. This web site might be one such place to start to understand the ExcelXP tagset and sending formats to Excel.
http://support.sas.com/rnd/base/ods/odsmarkup/excelxp_demo.html
However, keep in mind that when you use ODS HTML or ODS TAGSETS.EXCELXP techniques to write files (files which Excel can open), you cannot write to preformatted, already existing, Excel workbooks.
cynthia