Use this window to get an overview of the functions
  assigned to product models. You can also assign functions to a product model
  in this window. All engineering functions must first be defined in Product Basic Data/Functions. 
The engineering function structure is a conceptual level of product
  construction. Functions can be arranged in a structure. Each function on the
  bottom level can also have parts and the corresponding part structures
  attached.  The purpose is to organize the product structure so that it is
  familiar to multiple disciplines and to help with the construction of a
  product without having to immediately decide all its properties.
  
A function is the intended use of connected part(s). One part
  can be connected to more than one function, i.e., it can have several
  functions in the product structure.
  In Function Structure Navigator
  you can get a graphical view of the function structure.
Register Product Model Functions