As I continue to research this concept and do more thinking about it, I will give updates about valuable things I learn, or questions I have.
In the meantime, check out this quote from a book I've been perusing. If you're thinking about what kinds of materials to use in the classroom (for lessons, instruction, assessment, activities, etc), here's something to remember:
"... The kind of learning material to look for is that which is just within but at the outer edge of students' current abilities."
With this is mind, I can remember to keep students striving for the next level in their understanding and capabilities.