APPEND structure and Customizing Include are meant for enhancing the Standard Tables/Structures in customer perspective.
Normal Include structure is used to add fields to Z Tables.
APPEND structure : An append structure is a structure that is assigned to exactly one table or structure. There can be more than one append structure for a table or structure.
Append structure is used to add additional fields to standard SAP tables.
Append structure cannot be used with cluster and pooled tables.
Append structure is included in a table/structure using .APPEND
The name of a append structure can begin with Z* / Y* .