• Gharīb muṭlaq / fard muṭlaq: just one Successor transmits in the Successors' generation.
• Gharīb nisbī / fard nisbī: transmission is not limited to one narrator at the start, but a later generation has just one.
• The key difference: does the single narrator occur at the start of the sanad or later?
In this division, the start of the sanad means the Successors' generation.