Screencastify records the browser tab on which it was started, and warns you if you open a different tab. But when starting Screencastify, you have the option to record the desktop rather than an individual tab. This feature is experimental, but it worked fine for me. In addition, Screencastify animates the cursor and provides sounds for mouse clicks, but only when recording in tab capture mode. Overall, it is an excellent screencasting instrument for beginners, Chromebook users, or those who don't want to pay hundreds of dollars for screencasting software.
Snagit by TechSmith, the company who gave us Camtasia - the mother ship of all screencasting software, is also an excellent tool for screencasting novices or Chromebook users. The Snagit app is used for screencasting, and the extension for screenshots. But remember, both must be installed for proper functionality.
Snagit operates in much the same way as Screencastify. In fact, some of the windows in the two are identical. I often wonder if they are "twin apps from different programmers"! While Snagit tells me to select either a window or the full screen for recording, I only see the option for full screen. I see the exact same window in Screencastify, but, remember, it does allow me to record my desktop in a previous step. Both programs refer to recording as "sharing" the screen, and both show exactly the same window if you cancel the sharing. Sometimes I forget which app I am using...sort of a supernatural experience I guess. When I first started using Snagit, my screencasts did not appear on my screen when I stopped the sharing, nor were they in the Snagit library, as advertised. I uninstalled and reinstalled both the extension and the app, and the problem was solved. Those minor problems, plus the fact that Snagit has fewer features and options, makes Screencastify my choice when I'm creating screencasts with a Chromebook.
I need to point out, however, that neither Snagit nor Screencastify have editing capabilities. If you make a mistake in your screencast, you will probably need to do it over unless you can effectively edit it out with video editing software. So while it's great to finally have screencasting tools for the Chromebook, I use Camtasia for Windows PC to do screencasts. But if you need to make a video tutorial about using the Chrome Operating System on a Chromebook...now you can!