External resources
An resource is called "external", if it can't be dumped and restored without some help from the caller. Usually this mean that a part of it state is out of a dumped container.
Dumping
Typically external resources are just told to be such on dump. The respective option looks like --external TYPE[ID]{:VAL} option, where TYPE is resource type, ID is resource ID and :VAL is optional argument.
Restore
Restoring external resources heavily depends on its type. Some resources can be restored by CRIU itself with the help of --external option, some cannot. See below for further details.