The HDEPEND variable is used to represent build-time host dependencies. For build-time target dependencies, use DEPEND (if the host is the target then both HDEPEND and DEPEND will be installed on it). For EAPIs that support HDEPEND, the emerge --root-deps option will have no effect, since it is not needed when build-time dependencies are correctly specified with HDEPEND and DEPEND. If ebuilds using EAPIs which do not support HDEPEND are built in the same emerge run as those using EAPIs which do support HDEPEND, the emerge --root-deps option will only apply to the former.

The special "targetroot" USE flag will be automatically enabled for packages that are built for installation into a target ROOT, and will otherwise be automatically disabled. This flag may be used to control conditional dependencies, and ebuilds that use this flag need to add it to IUSE unless it happens to be included in the profile's IUSE_IMPLICIT variable.