俱舍論記 17.67

17.67

「八俱轉者至故無九十」者，明八業道與思俱轉。八俱轉者，謂先加行遣使作餘六惡色業，自行邪欲足前為七。行邪欲時必定有貪，故論不說，足前為八俱時究竟。後三自力必不俱行，故無九業道、十業道與思俱轉。

Regarding "Eight functioning simultaneously up to therefore there are no nine or ten": this clarifies eight courses of action functioning simultaneously with volition. 'Eight functioning simultaneously' means that through prior preparation one dispatches emissaries to perform the remaining six evil material karmas, and personally engages in illicit desire, which adds to the previous ones to make seven. When engaging in illicit desire there is necessarily greed, so the treatise does not mention it, which adds to the previous ones to make eight culminating simultaneously. Because the latter three cannot possibly operate together by their own power, there are no nine or ten courses of action that function simultaneously with volition.