Although many of Vim's registers support yank and paste behavior, they are limited to text that is inside of Vim itself. The selection registers provide access to the system clipboard, so that Vim and other applications can interact. The behavior of these registers depends upon many factors, such as which operating system is used, whether or not a mouse is used, etc, which can require more details than we can get into in this chapter.
More information about these registers can be found in the help pages by executing :h registers
.