大般涅槃經義記 6.9

6.9

復前六中，初五就其現在觀空，後一就其過去觀空。現中初三就內外事以觀空理，後二就其有為、無為法相觀空。

Further, within the first six: the first five contemplate emptiness in terms of the present; the last one contemplates emptiness in terms of the past. Within the present: the first three contemplate the principle of emptiness in terms of internal and external matters; the latter two contemplate emptiness in terms of the characteristics of conditioned and unconditioned dharmas.