In an effort to achieve standardization objectives for purchases of products used by the military and other government organizations, the U.S. Department of Defense created standards that these products must meet. You will see these standards called military standard, MIL-STD, or MIL-SPEC. These standards ensure products meet certain requirements, commonality, reliability, compatibility, and similar defense-related objectives.
The Department of Defense also has standards for two-way radios and radios that meet this standard must meet levels for such criteria as vibration, rain, salt air, sand/dust, shock (dropping), humidity, temperature as well as other factors.
Buying a product that meets mil-spec standards means you are buying a quality product that is built to withstand tougher use.
You may also see radios with an IP designation. IP stands for Ingress Protection and it is an international standard that specifies the sealing effectiveness of the enclosure of a product. It is a two digit code that tell you how effective the enclosure is to penetration from fingers, dust, and moisture.
For instance, for a radio that meets IP 55, the first 5 means it is protected from the amount of dust that would interfere with its operation. The second 5 means that it is protected from water that shoots from a nozzle.