Different applications contain different types of data, and they might make that data available in a variety of formats. gtkmm calls these data types targets.
For instance, gedit can supply and receive the "UTF8_STRING" target, so you can paste data into gedit from any application that supplies that target. Or two different image editing applications might supply and receive a variety of image formats as targets. As long as one application can receive one of the targets that the other supplies then you will be able to copy data from one to the other.
A target can be in a variety of binary formats. This chapter, and the examples, assume that the data is 8-bit text. This would allow us to use an XML format for the clipboard data. However this would probably not be appropriate for binary data such as images. Gtk::Clipboard provides overloads that allow you to specify the format in more detail if necessary.
The Drag and Drop API uses the same mechanism. You should probably use the same data targets and formats for both Clipboard and Drag and Drop operations.