The Compose bar is a very simple scratch pad. It consists of a textbox and two command buttons.
The textbox contains what ever you type into it. The first time you use the textbox it will be empty. The text you type into the textbox remains there until you change or clear it or you close the document.
The Clear Contents command button does as the name implies. If you click this button, Adapt It removes all the text from the box. If you clear the textbox you cannot restore the text. You must type it in again.
The Select All command button highlights all of the text in the box. Selecting the text does not place the text into the Clipboard.
Once you have selected the text, you can place it into the Clipboard by using the Cut or Copy commands on the Edit menu. You can also use their shortcut equivalents Ctrl+X or Ctrl+C.