It was a fine day and Radan was thinking about how to make the day even better.
Radan: I'll go grab lunch.
A wild sales person appears.
Sales person: Hey, quick question. What exactly happens when I edit the company and change its business type to Enterprise, I remember you guys saying you tied some logic to that.
Radan: Hm, yeah, I'm not sure, give me a moment, I'll check in the code.
Radan (thinking): Let's see ... hm ... and these callbacks run when ... why is this done like this ... what idiot wrote this ... oh, me ...
Radan (20 minutes later): Here's a list of all side effects. I'm mostly sure that's it.
Radan leaves, questioning his life choices.
Business/Domain logic encodes the real world rules of your business domain.
Business logic that changes
the state of the system