I see the 12th man was used in the Ashes today and he was allowed to bat for Australia. I have read the rules and this is not right according to the Rules. Has something been changed within the past few days.
I presume this is in relation to the use of Marnus Labuschagne as a substitute batsman for Steve Smith during the second test of the 2019 Ashes series at Lords. In which case this is an example of a Concussion Substitute.
The concussion substitute was introduced by the ICC to all forms of international cricket (Men's and Women's), and also to First-class cricket, from August 1st 2019 (the start of the new ICC World Test Championship).
It is designed to allow a team to make a "like-for-like" replacement (who can bat and/or bowl), that has been approved by the match referee, for a player that has been forced to retire because of concussion.
The rule can be found within section 1.2 of the ICC Playing Conditions:
If a player sustains a concussion or suspected concussion as a result of a head or neck injury during the course of the relevant match, a Concussion Replacement may be permitted in the following circumstances:
22.214.171.124 - the head or neck injury must have been sustained during play and within the playing area described in clause 126.96.36.199 above
188.8.131.52 - The Concussion Replacement Request must be submitted within 36 hours of the incident specified in 184.108.40.206.2 if a Concussion Replacement is to be permitted
The ICC Match Referee should ordinarily approve a Concussion Replacement Request if the replacement is a like-for-like player whose inclusion will not excessively advantage his team for the remainder of the match
Once the Concussion Replacement has been approved by the ICC Match Referee, the replaced player shall play no further part in the match
Above can also be found in the ICC Guidance Notes