Use of Extend question?

In the example:

=Extend([1,2,5,20])

What is the relevance of 1 and 2? Extend begins by looking at the slope of the last two elements. The other elements are not used/referenced by the model. They seem to be here merely for some sort of reference for the user/modeler.

Just want my assumptions confirmed. If this is not the case I am missing something.