I have been speaking to a number of people over the last few weeks about the new version of Dynamics AX (AX7) and some questions have come up about customisations and keeping them when upgrading. This blog from Joel Leichty @AXManuels answers some of those questions by explaining Packages, Extensions, Layers and Models which are important concepts in the new version.
Take a look!
An Extension is a brand new AX object that references an existing object. This style of development allows existing objects to be customized without directly touching the original object. It is a beautiful concept for application updates; upgrades become much simpler by reducing the number of dependencies.