Since this is pretty confusing, and becoming more and more important I thought I'd give a couple of examples using planet members and their fictitious animals
Red and the bull with squishy testicles ( an American fable)
Red has a very famous hot ticket show bull who has won everything - she has sold billions of units of semen on him (she calls him Sir), but unfortunately Sir has a personal problem - everytime he is collected he is either shooting blanks or dead sperm - Red is beside herself - the semen buyers are calling at all hours disturbing her beauty rest - shat is Red to do?
Red's vet - Dr Green - examines Sir and decides he has a bad infection of the testicles (called orchitis - don't ask me who calls bull balls orchids) - there are no approved drugs to treat orchitis in bulls. What is Dr Green to do?
Fortunately the FDA in it's infinite wisdom has created ELDU - so following the law Dr Green finds an appropriate antibiotic (Wondercillin) for Sir, he determines the withdrawal time based on science, he writes it all on the label and follows all the rules. Sir gets better, semen goes world wide, little Sirs spring up everywhere and Red retires to reign over the planet in recumbent splendor.
Now - if Red had a bottle of Wondercillin and gives it to Sir without following the ELDU she is technically breaking the law. This would become an issue esp if Sir was sent to slaughter (could save those little nuggets) and he sent the Wondercillin meter off the charts.
Now imagine Dr Green is really Dr Creep and he tells Red to give Baytril - did he break the law? YUP, there is no extralabel use of Baytril - Red would probably be in the clear if she was following written orders from her vet.
So in essence Dr Green can treat Red's bull with any human or animal drug - used extralabel (since there are no drugs labeled for his condition) as long as ELDU and AMDUCA are followed. Confused , yeah, we all are - but common sense, ethics, and a little knowledge will go a long way.
That's all for tonight but stay tuned - next we may have Jill's lame calf, knabes joint infection (not his joint) or shortdawgs cows metritis, or maybe OH B calf with a broken leg.....