Affordance

It's a bit like asking about the difference between theory and practice. You can have an element whose affordance is a priori correct but which isn't correctly interpreted by users. In UX, it's perception that counts. So we make sure to test design proposals with real people as much as possible.

Affordance can't really be controlled. An element of your interface will either suggest its use correctly to a user, or not. The signifier is there to give the user additional cues. An underlined interface element will suggest that it's clickable. Adding a cue such as an external-link icon makes it clear that clicking will redirect you to another site.

Affordance is worked on in the background. The thinking begins as soon as you work on the journeys and the intentions you want to highlight in your service. The choice of components, the care given to their various states, the arrangement of elements on the page or the visual identity you use are all elements that influence how your services are understood. If you work with a design system, you can review the components and the usage rules put in place. Once the journeys are designed, it's still worth double-checking that there's no interference during navigation.

For example, you very commonly come across an affordance problem in forms. Radio buttons are, by convention, associated with single choices. Checkboxes, for their part, are reserved for multiple choices. Your users will interpret what you expect of them based on these conventions. If you swap the two, errors will occur and neither you nor your users will be satisfied.

In reality, affordance is assessed locally (component by component) rather than globally and in a unified way. For example, on a mobile interface made up of different elements, you mainly look at the affordance of the elements that are interactive (e.g. buttons, calls to action, clickable links, or any other navigation element). From then on, what matters is checking that, for each of these elements, the intention you're offering the user seems obvious to them.