Commercial gastronorm pans are used at various stage throughout food preparation and service. They are an industry standard of pan sizes set by the European Standards Committee, as a way to make the purchasing and usage of cooking pans simpler and more interchangeable.
Often referred to as Gastronorm, GN or G/N, they typically come in both metallic and plastic variations, for use with hot or cold foods respectively. The sizing is based on the outer dimensions of the pan, and different equipment will sometimes state the size and variations of gastronorm pans they can hold.
Pans can come in different depths, too - ranging from as low as 20mm to over 200mm deep.
Not all equipment uses gastro norm sizing, however - so be sure to check your equipment first to make sure it's compatible.