When you create a Learn REST integration you see these options - End User Access - Authorized To Act As User. What is the difference? If you've been working with REST in Learn for some time, you know that End User Access has been available since we enabled Three-Legged OAuth (3LO) Checking End User Access allows the user associated with your REST application to act as the user who logs into your application using the Three-Legged OAuth process.
The new kid on the block, Authorized To Act As User, allows you, the Learn Admin to set the system to bypass the "Blackboard User Authorizes, User Authorizes" steps shown on the Three-Legged OAuth diagram. Why would you do that? There are cases, say if you're a copier company and your copier determines whether the user valid and authorized, the the REST Application running on the copier may need to bypass these steps, because the way some copier's "browsers" are built they can't follow the redirect.
And finally, for completeness, to answer the Partner who reached out about these final questions:
Also, once I specify a 'Learn user' in the REST API integration, are all REST calls performed as that user?
When using OAuth2 (Basic), yes.
Is there anyway to choose at runtime which learn user to perform the REST calls as?
You can't do that. You can only use 3LO to have the user log in, then all REST calls are made as if they were made by that user.
Reference http://oauthbible.com/ . What we've called "Basic" is two-legged.