AddIn (Add-In or Add In) is a synonym for plugin - a program that is triggered by a host program allowing third party developers to manipulate or extend the hosting program.
Visual Studio Tools for Office (VSTO) is a set of development tools for creating Office automation solutions (add-in or document customization), using .NET and the Visual Studio IDE.
"However since i know that the add-in is more or less working as is for all users some of whom use excel 2010 and some of whom use excel 2013 and some of whom may explicitly have vsto installed and some of whom may not i d like to be sure that i m not setting myself up for a bruising later on when some features of the add-in stop working for no apparent reason due to changing the setting"
from question "Implications of ignoring CS1762 ("A reference was created to embedded interop assembly ... because of an indirect reference ...")"
"The comaddin base class makes it a bit easier to create an office add-in and reduce infrastructure code to be more familar with vsto - thats the whole idea"
from question "NetOffice - IDTExtensibility2 vs ComAddin"