1. According to wall thickness grade (most important classification):

Schedule 40: Standard Wall Thickness. The most commonly used, suitable for the vast majority of indoor exposed, concealed, and partially buried underground (in compliance with NEC regulations) situations.

Schedule 80: Thickening the wall thickness. Higher mechanical strength, pressure resistance, and corrosion resistance. Suitable for high-risk areas, places susceptible to physical damage, or direct burial and outdoor exposure environments that require higher levels of protection.

Schedule 120: Wall thickness between 40 and 80, no longer commonly used.

Type A&Type B: This is the classification method in UL 651, typically corresponding to Schedule 40 (Type A) and Schedule 80 (Type B).

2. Key parameters:

Size range: The standard covers various nominal sizes ranging from 1/2 inch (16mm) to 6 inches (150mm).

Color: Usually gray. Sometimes other colors are also used (such as white for specific purposes), but gray is the standard color.

Marking requirements: The standard stipulates that the conduit must be permanently marked with the manufacturer's name/trademark, material type (such as PVC), size, wall thickness grade (such as "Sch 40"), UL mark, and standard number (such as "UL 651") at regular intervals (such as 1 meter).

Accessories: The standard also covers supporting accessories such as elbows, tees, straight through fittings, plugs, junction box connectors, etc., ensuring that their materials, dimensions, and mechanical properties match the conduit.