What is the meaning of hierarchy in architecture?

Gradation is the distribution of aggregate particles at all levels, which can be determined by screening test. Gradation in buildings generally refers to the gradation of sand and gravel. In civil engineering, aggregate with particle size greater than 5mm is coarse aggregate, also called "stone"; Aggregate with particle size less than 5mm is fine aggregate, also called "sand". Through sieve analysis, the size and collocation of sand can be calculated, and the gradation and fineness modulus of sand can be judged. Clinkers in the project are mixed in different proportions according to different gradations. Just like a bowl of stones, you can also fill it with coarse sand. You can also fine sand when it is full, and then you can add water. Its principle is to fill the gap with materials with different particle sizes to enhance its compactness and ensure its strength.