Here’s an excellent article by Borgland on the Aniyata (“Undecided”) rules in the Vinaya.
2. Borgland_Undetermined Matters BLS Vol II - 2016-2017.pdf (874.4 KB)
There’s a number of interesting points here. The rule allows for a case where a monk can be subject to a legal penalty (for sexual activity) solely on the word of a laywoman. As such it seems to contradict the general idea that monastics must confess before being subject to Vinaya penalties.
The Pali Vibhanga walked back this, but in this it turns out, it is alone. The article looks at how the rule is handled in different Vinayas, and shows that it is not always accurate to treat the Pali as a standin for early Buddhism generally.
I’ve written an article on the implications and evolution of this rule for Buddhadharma magazine, it’ll be out shortly.
Another issue here is how the Sarvastivada version of the rule expands the notion of “undetermined” to include anything undetermined, not just sexual accusations. This shows how free the early Indian schools were in commenting and interpreting the shared body of rules.