From OFLB

While regular font files are great, it is important to share your extended font sources files too as much as possible.

See the foo-open-font-sources DVCS template for practical recommendations about the various formats and best practices on branch and tarball structure.

Source code files are always available for free software programs, but they’re sometimes not included with free software fonts. But source files give the same benefits to type designers as regular program source code does for software developers – the ability to enhance things and share the improvements. The Open Font License doesn't require that source be included, but the GNU General Public License does.

A font source file is typically a SFD or UFO file produced by a font editor. It could also be a Python script used to develop the OpenType font, the files used for interpolation or multiple master outlines, Graphite feature files or OpenType layout table scripts, scans of drawings on paper, or anything else used to develop the font.

We strongly recommend using free software programs to develop your fonts. FontForge, one of the most popular font editors, creates SFD files by default. These are text files that are both human- and machine-readable. The UFO format comes from the RoboFab project, and uses XML files that are also both human- and machine-readable. Both formats are ideal for font sources because they are publically documented, so any font program developer can implement support for them. Practically, they also work very well with version control programs, too - and we recommend designers spend an afternoon to learn about version control and keep all their project folders in a version control system.

Proprietary font editors produce non-standard, binary files by default. For example, FontLab uses its own VFB format, which is not interoperable with other font editors. RoboFab can convert FontLab VFB files to UFO, and UFO to VFB; and FontForge has support for UFO. Robofab provides ways to convert a VFB. The next version of FontLab will move to open formats and provide import/export filters. The FontLab makers have made an interoperability promise at the last Robothon.

This page is related to the uploading page with more details about this topic.