"Click-to-Run" and "App-V" are MS names for the concept of a "virtual computer". Another name for this is a "sandbox". A term often used by modern browsers.
Basically, what happens is that inside the Q: drive / Click-to-Run / App-V a whole separate software only computer is created emulating all of the hardware and software needed to run a computer program. This isolates any program running inside of the "virtual
computer" from programs running on the "physical" computer. The most obvious example of this program isolation is that MS advertises that you can now install an Office 365 trial that includes Outlook on a computer with an older version of Outlook (you still
can only have one version connected at a time to the MS Outlook servers).
"Virtual Computers" provide a number of features / advantages:
- the person (ie MS) who sets up the virtual computer has complete control over the "environment", ie control of the simulated hardware and operating system (sounds like Apple ... hmmm)
- isolating a program inside of a "virtual computer" provides a degree of security from malware. This security goes both ways, protecting the virtual from physical, and visa versa
- MS has setup this virtual environment to bypass the Windows Update process. All Office updates inside of CTR are "pushed" from MS to the users, at MS's convenience (and sometime much to the inconvenience of users when the "update" is broken!)
Apparently the drive letter was not an absolute (no mention of "laws") requirement of Windows, but simply result of the way that MS decide to define the 2010 Starter App-V / CTR. As a result of the complaints from 2010 Starter users, they changed App-V for
2013 so that it no longer displays a drive letter.