After reading  several times, I’m realizing I don’t exactly understand what this “Add API Application” menu is all about.
It certainly gives a valuable insight on how grants are handed but not much about architectures.
Does it allow a FreePBX admin to implement:
a custom web application hosted on some local or remote host (ie not hosted on the same box FreePBX is installed on) and share credentials with FreePBX but not linked with FreePBX (a click within FreePBX web app does not link the this custom web app).
a custom web application hosted on the same box FreePBX is installed on and share credentials with FreePBX
a custom web application that queries FreePBX data
a custom web application queried by FreePBX when some events occur (extension creation, …).