I'm going to develop an Add-in (or Extension?) for ArcMap that will include a custom non-Modal window opened by clicking a button on toolbar to enable the user entering some data and interact with the map and layers extracting geometries and perform some geometric calculations.
Regarding the above I have two questions:
1. Would the above description require an Add-In or an Extension? What are the differences and limitations of one and the other?
2. What would be a better approach, to use ArcObjects or ArcGIS Runtime?
Also, since there is no ArcObjects help available on the ArcGIS for Developers, I was wondering if ArcObjects are getting deprecated?