عِبَارَةٌ عَنْ جَمَاعَةٍ اشْتَرَكُوا فِي السِّنِّ وَلِقَاءِ الْمَشَايِخِ
A ṭabaqah is a group of transmitters sharing a comparable age or period and encounters with teachers.
Being born within a single century is not sufficient by itself. Transmitters of similar ages might have studied in different regions under wholly different teachers; their actual transmission history matters.
Sinn refers to approximate age or era, while liqāʾ al-mashāyikh refers to encounters with the relevant generation of teachers. That combination is a useful clue when investigating continuity in a chain.