The description of sxg format expansion
The sxg file is a large document created by Apache OpenOffice, a free, cross-platform productivity application suite. The file may contain a table of contents, page text, formatting, graphics, spreadsheets and other document materials. Typically, the sxg file is used for storing books or long reports.
Master documents are useful when the file size of normal documents (sxw files) becomes too large for the computer to handle. They allow smaller documents to be referenced so they are not loaded into the program. Typically they are only used by advanced users. Currently Apache OpenOffice uses ODM files for master documents instead of sxg extension.