The FEATURES variable specifies options which affect how Portage operates and how packages are compiled. It is not used for settings which have a substantial effect upon the resulting generated package.

Relevant FEATURES for developers include:

Feature Explanation
collision-protect Raise an error if an installing package attempts to overwrite a file which is provided by a different package.
noauto When utilizing ebuild, only run the function requested.
sandbox Enable the sandbox.
sign GPG sign Manifest files.
strict Do some extra checks for potentially dangerous situations (eg missing Manifest files).
test Enable the src_test phase.
userpriv Drop to non-root privileges for certain phases.
usersandbox Enables the sandbox even when running non-privileged.