The 80/20 principle also known as the Pareto principle applies to many areas.
The most common application of the principle is in the assessment of a project effort: 20% of the effort produces 80% of the job to be do done. Or the other way around, the last 20% of the work to be done will take 80% of the effort.
In IT, the principle applies also similarly to requirements versus functionality: 20% of the requirements determine 80 procent of the architecture. 20% of the requirements are the important ones for the core construction of a solution.
The principe thus tells you to focus on the 20% important requirements that determine the architecture. It helps you shrink the options you need to consider and prioritize and focus on the important parts of the solution.
The question is of course: which of the requirements are the important ones. The experience of the architect helps here. But in general you will realize while analysing requirements, if a requirement will need a significant change or addition to the solution.
A good book about the 80/20 principle is the book with the same name: The 80/20 Principle, by Richard Koch.
An example from an practitioner architecting an airline reservation system.
“The first time I (unconsciously) applied the 80/20 rule was in my early days as an architect. I was working with a team of architects on application infrastructure for completely new web applications. A wide variety of applications were planned to run on this infrastructure. However, it was not clear yet what the characteristics, the volumes, response time needs, concurrent users et cetera were for these applications, and that made it uncertain what we needed to cater for in this generic hosting solution.
So we decided to walk through the known use cases for the different applications.
We worked our way through four of the tens of applications. During the fourth we somehow could not come up with additional requirements for the application infrastructure. We realized that the rest of the set of applications would ‘only’ be a variety of one of the apps wealready looked at. So we had our 80% from looking at just 20%.”