It is there, important to obtain expert assistance for all serious conditions. One can also prescribe historically, that is for something which occurred in the past, even years in the past, but which is felt to be the cause of the present ills. This might be an accident, the loss of a favourite companion, an illness which was perhaps only suppressedand not cured by conventional treatment or which, although apparently cured, has left the patient unwell . This is not an easy task and generally requires an advanced degree of knowledge. Giving the incorrect remedy won't harm the patient but it won't effect a cure either, which is why all serious conditions should be referred to a vet.
Some books give the potencies which have been found to work the best or are thought most suitable.
Low potencies have a wide range of effect, but do not go deep. While high potencies are deep acting but have a very narrow range. This means that a low potency of a not quite correct remedy will probably have some effect on the symptoms, but a high potency needs to be of exactly the right remedy to have any effect at all. Equally, a low potency may not be strong enough to effect a complete cure even if it is of the right remedy. As a rule of thumb, in urgent acute cases unless one is absolutely sure of the remedy choice, use a low potency to get the width of effect.