The customer is page because it requires only one embedded page.
The addresses is a page group because as we can have multiple addresses like the address for home, office, etc, the subscript is required to identify the type of address (Page group is an unordered list)
The line item is a pagelist because it does not require any subscript to identify the type of line item as the same type of items can be multiple and it can be an ordered list (Pagelist is an ordered list)
A Page List property is numerically ordered list of embedded pages that are referenced by a numeric subscript, starting with 1. Page List properties are useful for holding lists of data.
Page List Items will be referenced as .LineItem(1), .LineItem(2) and soo on
A Page Group property is an unordered set of pages that is referenced by a string subscript. Page Group properties are useful when you need to look up specific pages by a unique identifier and the order of the pages does not matter.
Page Group Items will be referenced as .Address(Home), .Address(Office) Etc.