Al-sābiq wa-l-lāḥiq refers to two transmitters from the same teacher whose lifetimes or death dates are separated by an unusually long interval. They need not have been peers who studied together: one may have heard the teacher early in his life and the other late.
The example involving the long-lived scholar al-Silafī illustrates how widely separated the periods of pupils of one shaykh can be. Research implication: a claimed direct transmission across a long interval requires checking dates, lifespans and opportunities to meet.
This category is neither a synonym for high isnād nor for mudabbaj. Its central question is the temporal relation of early and late pupils to one common teacher.