律戒本疏 1.20

1.20

第二因利比丘與女人露坐語　居士薩薄呵見而嫌之。即以白佛。以露處亦多人所見。必致譏嫌。是亦不可。雖母姉妹是亦不得。人不達故。律師儀云。所以在二篇之後三篇之前。特以不定輕五篇故受其中也。意以為上二篇罪重說之以竟。若有犯者宜其諦思。故說不定之篇令其首定。

Second: Originating from an undisciplined monk sitting and talking with a woman in an open place. The householder Sarvada saw this and disapproved of it, and immediately reported it to the Buddha. Because an open place is also seen by many people, it will inevitably incur criticism and suspicion, and this too is impermissible. Even if it is one's mother or sister, this too is impermissible, for ordinary people do not understand. The Vinaya master Yiguan states: The reason it is placed after the two classes [pārājika and saṃghāvaśeṣa] and before the three classes is specifically because the undetermined rules deal lightly with the five classes, and are placed between them. The intent is that since the grave offenses of the first two classes have already been explained, if anyone violates them, they should contemplate carefully; therefore, the undetermined class is expounded to cause them to confess and resolve the matter.