Function libraries and interfaces
Not every function in Microsoft Dynamics NAV has a clear owner. In some cases, we have a function that can be accessed using simple variables that perform some calculation or a small piece of business logic.
These functions can be put in function libraries or interfaces. By doing this, it is easy for other developers to reuse them.
Examples of function libraries can be string manipulations or date functions. Microsoft Dynamics NAV does not have a lot of built-in functions for this, and many developers have created their own collection of functions through the years.
An Interface is a reusable component that can be accessed from anywhere in the application. A very good example of what could and should be an interface is the function called
DateNotAllowed in Codeunit 11, Gen. Jnl.-Check Line:
IF (AllowPostingFrom = 0D) AND (AllowPostingTo = 0D) THEN BEGIN
IF USERID <> '' THEN
IF UserSetup.GET(USERID) THEN BEGIN...