A rule is some normative command that has some universal application. However, every action is itself particular. The rule in itself as applied to each individual can only be analogous taken in each case. For example, the medical profession has the norm of “Cause health, not harm.” However, health in itself is an analogous term which differs when considering feeding someone versus applying medicine versus amputating a limb to save the person.
The rule may be more and more refined, but the more refined it becomes the less of a rule it becomes. As it approaches the level of the particular action it takes on the character of a narrative rather than a rule. That is it becomes a description of a series of events which is necessary inapplicable to other situations where the narrative is different.
However, some normative rules are univocal in all case, because they concern thing that are intrinsically good or evil. “Do not murder” is clear in all cases, because the term “murder” is univocal in all cases.
Yet, not all ethics can be reduced to such univocal rules, because not all situations has a clear answer phrase in some universal and univocal judgement, consider medical example above.
In light of this, in order to know when and how to apply these analogous rules, we require some overriding and well-formed judgement. This judgement proceeds from the practical intellect, the perfection of which is called the virtue of prudence. Hence, the application of deontological rules requires the virtue of prudence.