The rules of discipline first laid down by the Buddha are called Mfilapanizatti (the root regulation); those supplemented later are known as Anupaňňatti. Together they are known as Sikkhāpadas, rules of discipline. The act of transgressing these rules of discipline, thereby incurring a penalty by the guilty bhikkhu, is called Āpatti, which means ‘reaching, committing’.
The offences for which penalties are laid down may be classified under seven categories depending on their nature
An offence in the first category of offences, Pārājika, is classified as a grave offence, garukāpatti, which is irremediable, atekicchā and entails the falling off of the offender from bhikkhuhood.
An offence in the second category, Saṁghādisesa, is also classified as a grave offence but it is remediable, satekicchā. The offender is put on a probationary period of penance, during which he has to undertake certain difficult practices and after which he is rehabilitated by the Samghā assembly.
The remaining five categories consist of light offences, lahukāpatti, which are remediable and incur the penalty of having to confess the transgression to another bhikkhu. After carrying out the prescribed penalty, the bhikkhu transgressor becomes cleansed of the offence.
Source: Guide to Tipitaka by Sayagyi U Ko Lay