When you apply a design template (design template: A file that contains the styles in a presentation, including the type and size of bullets and fonts; placeholder sizes and positions; background design and fill color schemes; and a slide master and optional title master.) that includes multiple slide masters (slide master: The slide that stores information about the design template applied, including font styles, placeholder sizes and positions, background design, and color schemes.), you get a message asking if you want all the masters in this template to be available for the presentation. Here are the results if you click Yes or No:
You can delete masters from within master view.