Adress of Elements in Assambly

Hello,

I have following challenge to clearify. To automate the interface between PTC Creo Parametric and external calculation tools I have to get access to several properties od the CAD model like dimensions or weight. For that I need to know how to pick these individual elements. 

Example: I want to know the adress of a diameter of a shaft, that is part of an assambly. Which adress does the diameter have? How to pick the this diameter from external programs?

I would be very grateful, if somebody can help.

Alex