The Extend Mode in Microsoft Word is an artefact of the early computing era before the mouse was invented (and the natural evolution of that gunky stuff that builds up on your mouse's bottom). Few computer users make use of this feature today because they've learned how to scoot around their documents with the mouse and on-screen scroll bars. But the Extend Mode can still come in handy every once in a while when you need to select sections of text.
This works in both Microsoft Word and Microsoft Outlook (if you use Word as your email editor). To enable the Extend Mode, press F8. Now try moving the cursor using the cursor control keys (not the mouse) and you'll see a selection appear from the point where you first activated the Extend Mode. If you wanted, for example, to select all the text from your current cursor position to the word "catapult" you would turn on Extend Mode by pressing F8 then press CTRL+F (for Find) then type in the word Catapult then press Enter. Finally press Cancel to close the Find box and your text is selected. To turn off Extend Mode simply press the Esc key.
Using the Extend Mode can feel a bit clanky at first but with practice it can be a useful extension (sorry...) to your keyboard and mouse navigation tools. Give it a try.