The general description of how selections and the clipboard works under X is given in clipboards.txt. FontForge tries to follow this.
FontForge only supports the PRIMARY selection within textfields. Text is exported as
Traditionally the PRIMARY selection is available for pasting by clicking on the middle mouse button.
After a Copy command FontForge will place data in the X CLIPBOARD. For text selections (in textfields) the above format types are supported. For contour data (ie. the contents of an outline character) the format "image/eps" or "image/svg" will be used. As a special case, if a single point is in the clipboard then it will also be exported as a STRING containing the coordinates of the point (to make it easier for people to identify points when sending me email, or in the comment field). For bitmap image data either "image/bmp" or "image/png" (assuming you have libpng on your system).