All apps need an API interface to communicate with third party...

All apps need an API interface to communicate with third party software. API stands for Application programming interface. Source here: https://en.wikipedia.org/wiki/Application_programming_interface

Different Apps have different API configurations and so not all apps will be compatible with all third party software.

For example, if an app does not have an API for "registering new users", then that app would be unable to communicate with a platform that requires registration and therefore that app would need to be manually added.

9 Spokes relies on a set of API compatibility, and if an app doesn't support the full set of API, then some features of that app will not be compatible or may not be compatible at all. Log on to 9 Spokes to see the current list of supported and unsupported apps.