Fard mutlaq: isolation at the originating or pivotal link; later branching does not automatically remove that status.
Fard nisbi: isolation relative to a particular transmitter, teacher, or relationship; the report may still be widely transmitted overall.
Fard and gharib: closely related technical terms. Fard is more commonly used for absolute and gharib for relative singularity, but this is a tendency rather than an invariable rule.
Tafarrada bihi / aghraba bihi: related verbs can occur in either type; the relevant isnad context determines their intended force.
The central objective is to identify the location, scope, and nature of isolated transmission. A different inquiry follows: under which conditions does an ahad report qualify as sahih li-dhatihi, and what makes it acceptable?