Escher is part of the Office 97 DLL. It replaces the existing drawing layers of PowerPoint, Word, and Excel. Clients access the Escher services through custom APIs and COM interfaces defined in msodr.h.
When serializing Escher data into a file, the client application provides an OLE IStream interface to Escher. This document describes the records Escher writes to this interface. However, it doesn't describe how the actual bytes are saved to disk. The client controls this format, since Escher writes to a client provided interface. Excel, for example, places the bytes of the Escher stream into BIFF records.