In the Generation II games, type data is represented by a byte. Only 28 of the values for type are defined, including the unused Bird type and the placeholder "Normal" types with index numbers between 10 and 18, of which 18 are used. The remaining 228 values for type are treated as garbage data.
A glitch type is not actually the name of a type. Instead, it refers to a type with no constant name, meaning that giving it a suitable name can be difficult.
The Normal type, also known by fans as the Abnormal type, is the type of several variations of ?????. Although it shares its name with a standard type, it has no strengths or weaknesses.