The file extension CEF is used to denote so-called compact embedded fonts, ie embedded fonts of the illustrator vector drawing software. CEF is an Adobe special technique that is optimized especially for the presentation of small fonts.
If fonts are used in a svg document, they can either be embedded directly or linked. When linking (linking) fonts, these are paged out in a CEF file, which Illustrator automatically generates when saving a svg document.
These CEF files can be used in other svg files and save space. Note that CEF is not part of the SVG standard and therefore svg files using this technology can not be viewed by any svg viewer. Such svg files can be displayed with the Adobe SVG Viewer, for example.