Gents,
I have been contacted by a person, who wishes to remain nameless, but who has performed the duties of No.4/5 in a professional environment numerous times.
They would like it stated that:
Replacements are NOT limited.
You may
substitute up to 2 front rowers and up to 5 other players. This is plain Law. The Law has already defined the difference in the Definitions.
The commenter Dixie has gotten closest in his or her(sic) post.
When the IRB says that 3.12 allows the sub, what they meant is that 3.12 allowed Robinson to return as a replacement. Therefore Australia had not used up its 2 permitted front row substitutions.
When Dixie asks were there actually 5 substitutions left available (given only 2 players were substituted, the rest for injury) - the answer is YES. But they didn't have 5 bodies left, so it's a moot point!
When the IRB say the Australians had one substitution left, what they really meant was that Australia could still do one substitution (since they had one substitute left). Fainga'a could have come on tactically.
You can make a maximum of 9 "movements" under current Law with a bench of 7 not including blood and YC/RC provisions (ie 7 tactical subs, plus 2 front row injuries). The teams know this, the managers know this, and every 4/5 worth his salt knows this. It's actually bloody simple ... but as soon as Rolland conflated substitutions and injuries, everyone who does not know what they are talking about has weighed in and confused the matter.
As final evidence that what is going on is common practice consider these two scenarios:
1) A team does all 7 tactical subs, including one prop and one hooker. There is 10 mins to go. Not implausible. A prop gets injured. If replacements and substitutions were all covered by a total of 7, you could NOT replace this prop with one of your perfectly fit front rowers sitting there on the bench, because you'd already used your 2 front row movements. We all know that this is ludicrous and such a move would never be blocked. It's just nonsense. Of course the move would be allowed.
2) South Africa starts with 3 front rowers on the bench. They have used this tactic in many Tests and also SA teams (and the Waratahs at times), have done it in Super Rugby. If replacements and substitutions were all covered by a total of 7, you could NEVER EVER use one of the 3 players, because you are only allowed 2 front row subs. What a stupid tactic that would be! It's not though, because that's not what the Law is! There is usually 2 tactical subs, and then an injury (or "injury"!) and the third front rower is used.