What is the meaning of number's written inside a recycle symbols

Plastic have available in different grade , for better understanding of user point of view manufacturing have print a symbol .

These symbols represent the grades of plastic 

All the image credit for Indiamart.

1. Polyethylene tetrapathalate

Easily recycled 

 

2. High Density Polyethylene

 

3. Polyvinyl chloride

4. Low Density Polyethylene

5. Polypropylene 

6. polystyrene

 7. Polycarbonate (others )