There are a number of cases where a complex shaped surface cannot be adequately textured from one photograph. By choosing the multiple-photo setting you can apply more than one photograph to a surface. PhotoModeler decides which photograph will be applied to which part of the photograph based on the angle of the surface relative to the photo and the distance. See the Advanced Materials Dialog for more information.
The photographs to be applied to the surface are defined by a Photo Set. For example, if the “All” Photo Set is chosen then any photograph in the project can be used for photo-texturing that surface (it does not mean that all photographs will be used just that any one or more of the set can be used). The Photo Set is then used to restrict the photographs you may not wish to have texture a particular surface. For example, if photo 5 is of poor quality, you can create a Photo Set with photo 5 missing and then use that set in this drop down list.
Since the Material name is shown on various dialogs and on the main toolbar Material drop down selector, it is a good idea to assign an appropriate name to a multiple photo texture material, such as the source photo set that you selected.