What are they and what’s the difference?

An add-in is a software program that expands the capabilities of bigger programs. It is a term commonly used by Microsoft and other platforms which have additional functions that can be added to primary programs.

An add-on is a software extension that adds extra features to a program. It may extend certain functions within the program, add new items to the program's interface, or give the program additional capabilities.

Expansion vs extension. There’s a difference, but it’s not very clear to me in the context of a software application. Both offer added functionality.

Such add-ins /add-ons may be provided by the main Application provider (such as Microsoft) or by third parties. I want to discuss add-ins / add-ons from parties other than the provider of the main application, because that is where some potential issues lie.