There are a number of layers - some are now obsolete but might still be utilized by intrepid CSG coders or Partners. The majority of the layers have a defined load sequence and Parent.
Base Extensions - first layer - This layer was designed to be used by Epicor development but has since been used by CSG and Partners. This is often used by Epicor to make a “tracker” from a data entry form. Parent is the UI itself and Base Extensions are not company specific.
Productization - second layer - This layer was also designed to be used by Epicor development and is now considered obsolete. Remember the good old days of Vantage 8 and Vista? Vista was a slimmed down version of Vantage and it was generally slimmed down by removing and moving controls using the Productization layer. Parent is the UI or the Base Extension. Loads based on the “Product” code (which used to be VN and VS and is now EP) and are not company specific.
Product Extension - third layer - This is another Epicor Development layer - it is to Productization what Base Extensions are to the Base. Used to further refine a Productization definition generally for Trackers. Parent is the UI, Base Extension, or Productization otherwise uses Productization load rules.
Verticalization - fourth layer - This layer was designed to be used by Partners or CSG. Implementation logic is a little odd and this layer is not as useful as it could be. I believe this loads against a Parent layer (not 100% sure) and it loads based on the presence of a Verticalization code in the DB and is not company specific (this is where the logic is a little odd as it really should be company specific).
Localization - fifth layer but with special rules - this layer was designed to be used by the Epicor Country Specific Functionality (CSF) developers. Unlike the previous layers, this layer does not have the concept of a “parent” layer so will load against a UI regardless of the presence or absence of any lower levels. Loads based on the CSF and/or Country Group Code (CGC) assigned to the Company. Original idea was that a Localization “package” could be made up of Localizations specific to a CSF (Finland) and also to a group of Countries (Scandinavia). Load Logic looks for CSF Localization and if not found, looks for the CGC Localization. The CGC code capability has never been used.
Customization - sixth layer - designed to be used by the Epicor Customers. Parent is the layer in play when the Customization is created. Can be company specific.
Personalization - final layer - designed to be used by the individual end users. Parent is the layer in play when the Personalization is created. Company specific.