What does it mean that there is a 5 on the bottom of the plastic lunch box?

"5"——PP (Polypropylene)

Common uses: special lunch boxes for microwave ovens, etc.

The microwave lunch box is made of this material and is resistant to high temperatures of 130°C. This is the only plastic box that can be placed in a microwave oven and can be reused after careful cleaning. It is important to note that some microwave lunch boxes are made of No. 05 PP.