Feature Symbols (named FEATURE_{feature name}, all caps) are compilation symbols defined for features whose availability vary across platforms that IronPython supports. The symbols are defined in Build/{framework}.props file, which get included by all .csproj files that contribute to IronPython.

The list of symbols currently in use is described in feature-symbols.md. This is a versioned document that changes together with the source code, so use the version from the same commit as the code you are working on.