What is the 3rd type of "scalability"

I read a little about horizontal and vertical scalability, but what is the 3rd type of scalability called.

Horizontal and Vertical scalability involve increasing the amount of machines or hardware power to allow for more users, the original code stays the same.

But what is it called when your app is scalable in such a way that you can easily add new features without breaking things or having to do a lot of work?

Is it just called best practices?

I’d like to know the name so I can find resources to learn about it. All my google searches end up showing results for Horizontal and Vertical scalability

Modularity? Software architecture? Clean Code?
Any of these may apply.

Googling making modular express app seems to have gotten me the types of results I wanted.