The term parameter is in general used for both the definition of the argument that should be passed to an Entry, program, procedure, function or subroutine and for the argument that is passed. This poses a problem.
If I define a mathematical function f(x) = x + 2, the identifier 'x' is said to be 'the parameter' of the function f, meaning that it indicates the argument that should be passed. If I write down an application of the function f, e.g. f(3) 3 is also said to be 'the parameter' of the function f, meaning that it is the argument that is actually passed to the function f. When I talk about 'the parameter' of the function f it is not clear whether I mean x or 3.
In order to distinguish between these two uses of the word parameter we call an identifier that is used in the function definition to denote an argument that should be passed the Formal parameter (x in f(x) = x + 2). An expression that yields the argument that is passed is called an Actual parameter (3 in f(3) or a+b in f(a+b)).
An Actual parameter for an Entry application can be provided in several manners.
If the Entry is a Main Entry, the actual parameters will be provided through Key Selection. The Key Retrieval method defined with the Entry is called. This method yields a set of actual parameters that will be used for the data retrieval method.
If the Entry is a Subentry, the actual parameters are defined in the Entry-Subentry definition.
Actual parameters can always be provided or overridden by the PAR keyword in a WITH or FORALL instruction in ITP Language.