In FontLab VI, composite glyphs (short: composites) are glyphs made up of two or more elements, of which at least one element is a reference. like a letter plus an accent. One or more of the elements are referenced. I.e. their contours are not actually present in the complex glyph, but are “copied” from and linked to some other glyph. Thus whenever the original element contour is changed, all the complex glyphs that copy the element also change.
This approach is a bit different to composite glyphs made up of components. You can edit referenced elements not only in the original glyph but also right in complex glyphs with links.
When the font is exported to the OpenType TT format, complex glyphs are transformed to composites. When the font is exported to the OpenType PS format, complex glyphs are used for CFF compression.
Elements may also contain bitmap color images that can be exported to Font Formats supporting color bitmaps.