My understanding is this: A skill is made up of SKILL.md which is what tells claude how and when to use this skill. I'm a bit of a control freak so I'll usually explicitly direct claude to "load the wireframe-skill" and then do X.

Now SKILL.md can have references to more finegrained behaviors or capabilities of our skill. My skills generally tend to have a reference/{workflows,tools,standards,testing-guide,routing,api-integration}.md. These references are what then gets "progressively loaded" into the context.

Say I asked claude to use the wireframe-skill to create profileView mockup. While creating the wireframe, claude will need to figure out what API endpoints are available/relevant for the profileView and the response types etc. It's at this point that claude reads the references/api-integration.md file from the wireframe skill.

After a while I found I didn't like the progressive loading so I usually direct claude to load all references in the skill before proceeding - this usually takes up maybe 20k to 30k tokens, but the accuracy and precision (imagined or otherwise ha!) is worth it for my use cases.