俱舍論記 9.134

9.134

「由中際廣至說便無用」者，答。中際相顯，廣說果因；前、後難知，所以略說。由中比二，廣義已成。

Regarding "Because the middle period is extensive [...] saying [it] would be useless": this is the answer. The characteristics of the middle period are manifest, so causes and fruits are expounded extensively; the previous and subsequent [periods] are difficult to know, which is why they are expounded briefly. Because the two [other periods] can be inferred from the middle, the extensive meaning is already established.