This is from Wikipedia, I don't know if this will help with your questions. At the end it explains the basic definition of a shared platform.
is a shared set of components common to a number of different automobiles. Many vendors refer to this as a vehicle architecture. Originally, a platform was a literally shared chassis from a previously-engineered vehicle, as in the case for the Volkswagen Beetle frame under the Volkswagen Karmann Ghia. The first generic platform to be shared among a number of vehicles was the Ford Fox platform of the 1970s. In the 1980s, Chrysler's K-cars all wore a badge with the letter, "K", to indicate their shared platform.
Today, platform sharing is much less noticeable. Vehicle architectures consist of "under the skin" components only, and shared platforms can show up in unusual places like the Nissan FM platform-mates Nissan 350Z sports car and Infiniti FX SUV. Volkswagen A platform-mates like the Audi TT and Volkswagen Golf also share much of their mechanical components but seem entirely different. Ford Motor Company has had much success building many well differentiated vehicles from many marques off the same platforms.
Key mechanical components that define an automobile platform include:
* Chassis, along with passive car safety features and key body dimensions
* Steering mechanism and type of power steering
* Type of front/rear suspensions
* Choice and placement of engine and other powertrain components