When you save your .fbx file using the File Save or File Save As options, you can select to either save your .fbx file as an .fbx Binary file format or an .fbx ASCII file format. Although the default save .fbx file format is binary, MotionBuilder saves to the lile format you last selected as the default.
While binary is the standard format for FBX files, .fbx ASCII format gives you a "plain language" version of the file format that enables you to search the file for information retrieval. You can specify ASCII format for OR SDK or Python files as well.