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.
Dump
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.
Resource types
The following external resources can be checkpointed and restored: