The definitions provided are tremendously valuable to figure out the very similar but different two paradigms.

Epics, issues, requirements, and others all have similar but just subtle enough differences in common interactions that the user needs to hold a complicated mental model of how they each behave.

I want to emphasize that Result is just an alternative name for the Either monad.

it is inconvenient to write specific implementations for each datatype contained, especially if the code for each datatype is virtually identical. For example, in C++, this duplication of code can be circumvented by defining a class template

An operation has two invocation styles. This is the only difference to an Activity.