1. A computer system comprising: a central processing unit (CPU); a memory coupled to said CPU; a computer-readable, tangible storage device coupled to said CPU, said storage device containing instructions that when carried out by said CPU via said memory implement a method of transforming a starting list of purchase types into a selection list of valid purchase types by employing a configurable rule, said method comprising the steps of: receiving, by a software-based common accounting module (CAM) executing on said computer system, a plurality of parameters of an item whose purchase is being charged to an account by an execution by CAM of an accounting function of a plurality of accounting functions, wherein said plurality of parameters includes a unit price of said item and a commodity code that identifies said item; generating, by said CAM, a starting list that includes a set of purchase types, wherein said generating is based on a pre-defined category of said item, wherein each purchase type of said starting list specifies a corresponding accounting function of said plurality of accounting functions; identifying, by said CAM and subsequent to said step of generating said starting list, a rule set of a plurality of rule sets stored in a rule set database table, wherein said rule set database table associates said rule set with one or more purchase types that specify one or more corresponding accounting functions of said plurality of accounting functions, wherein said identifying said rule set comprises determining that said one or more purchase types is a maximal subset of said set of purchase types included in said starting list; identifying, by said CAM and subsequent to said step of identifying said rule set, a rule stored in a rule database table, wherein said identifying said rule is based on said rule set and based on said unit price; and applying said rule by said CAM and subsequent to said identifying said rule, wherein a result of said step of applying said rule is a selection list comprising a proper subset of said starting list.