When analyzing a law, one can discuss the "subjects" of the law; e.g., the subjects of a law that imposes a sales tax will be the merchants and/or buyers. But how would one refer to the substance of the law itself as a distinct entity? Using my previous example, I'm looking for a word that would complete this sentence: "The __ of this law is the imposition of a sales tax." The word "substance" seems a little too broad for this purpose, as the subjects of the law are also part of its substance.
[Note: I'm well aware that the above sentence could easily be reworded as something like "The law imposes a sales tax"; it's just an example to explain what kind of technical term I'm trying to find.]