Refactoring to a really small but useful model island inside a sea of Legacy code

A few years ago the system I was working on involved a set of rules that allows for job applicants taking part of selection tests to make use of exemptions or (unfortunately) blocking them from taking further tests . This part of the system began as a set of queries against the main database, with no explicit domain knowledge in the code. As time passed.... this code proved difficult to reason about as requirements changed, new reasons to give exemptions or block application were introduced. In the end , it was refactored as a combination of strategy & composite pattern. The gain there was of course being able to add / remove new strategies more easily. The main point I got out of that refactoring though was we got explicit domain terminology , clear rules and an easy way to reason about the specifics of this island of better model in the code