How can we define ''Progressive Elaboration'' in context of PMI framework?

PMI website defines it as:

Progressive elaboration, is defined by PMBOK*as

“continuously improving and detailing a plan as more detailed and specific information and more accurate estimates become available as the project progresses, and thereby producing more accurate and complete plans that result from the successive iterations of the  planning process”.

Progressive Elaboration

- this concept is key to the entire PMI framework. It fundamentally states that

you cannot come up with a definitive estimate for timeline and budget at the very start of a project.

Why? The reason is that there are many unknowns and very little analysis has been done at the

beginning of the project. As the project team dives into the analysis, consults with subject matter

experts, and begins to define the details of the project - only then can more accurate estimates be

created.

The greater the number of the unknowns that exist in a project, the more a progressive elaboration is

required to ensure project success.