AQL inspection levels are separated into two categories: general and special. 

General inspection levels (GI, GII, GIII) and special inspection levels (S1, S2, S3, S4) indicate the size of the inspected sample batch, which determines the number of defects allowed based on product-specific criteria and tolerances. Special inspection levels call for smaller sample sizes, with S4 being the most stringent of the four options. 

Lumi prescribes AQL 2.5 Inspection Level S4 because it’s a happy medium for the quality control needs of packaging. This level allows for an adequate number of samples to be inspected without delaying the production schedule or the receiving process.