ABAP Implementation Class
The Implementation Class allows you to implement certain functionality in ABAP that will be called throughout the lifecycle of the application.
Once implemented (see Create an Implementation Class), the platform will automatically create and update a type declaration in your ABAP class that will represent your current data model. After any changes to the data model are made and the application is saved again, the type declaration will automatically get updated.
To manually regenerate the Type declaration, you can click the Generate GS_DATA into Class 
button in the Data Structure area of the developer workbench.
It is recommended that you select the Protected Section for GS_DATA in the Properties dialog and not create any manual protected methods. Doing so may result in the type declaration from being removed. In this case, you can use the Generate GS_DATA into Class 
button to update the class again once the method has been added and activated.