You have this on the wiki, I don't know if it is what you search.
# transaction (optional) is the required level transaction; valid values are:
* none specifies that no transaction is required at all
* required specifies that a transaction is required (and will inherit an existing transaction, if open)
* requiresnew specifies that a new transaction is required
* Note: if not specified, the default value is none if authentication is also none, otherwise the value is required