A slide layout is essentially a template design which can be applied to one or more slides, defining the default appearance and positioning of objects on the slide.

The appliance would be to load data from the slide layout in a first time.
In a second time, we load data from slide.xml and overwrite data from layout.

Links :

• http://officeopenxml.com/prSlideLayout.php
• https://msdn.microsoft.com/en-us/library/office/gg278311.aspx