UN number. The four-figure identification number of the substance or article taken from the UN Model Regulations
Unit load. Unit load means that a number of packages are either:
1. placed or stacked on and secured by strapping, shrink-wrapping, or other suitable means to a load board, such as a pallet;
2. placed in a protective outer enclosure, such as a pallet box;
3. Permanently secured together in a sling.
Upper explosive limit (UEL). The highest concentration (expressed in percent vapor or gas in the air by volume) of a substance that will burn or explode when an ignition source is present.