Slideout panels should be used for actions related to the user’s primary path. These panels allow for greater content and/or more complex interactions thus behaving much like an additional page while maintaining a contextual connection to the primary task.
Using of Modals
Modal windows are best used to focus attention either on some particular content (such as a video), or to confirm an action or decision (such as “Do you wish to delete the selected files”). Typically modals should be used when the information presented is more concise and the interactions are less complex (than that used in slideouts).