The 4 special inspection levels

[Tanvir Ahmed Chowdhury]

These special levels can be applied in cases where only very few samples can be checked. "Four additional special levels, S-1, S-2, S-3 and S-4 […] may be used where relatively small sample sizes are necessary and larger sampling risks can be tolerated."
Under S-3 level, the number of samples to check is lower than under S-4, and so on.
 
In practice: for consumer goods, quality control is usually performed under the general levels.
The special levels are used only for certain tests that either take lots of time or destroy the samples. Another situation where special levels are appropriate is a container-loading supervision--to have an idea of what is inside the cartons, without spending too much time at that checking.

[Debangshu Paul]

Good informative post