A type certificate is issued to signify the airworthiness of an aircraft manufacturing design. The certificate is issued by a regulating body, and once issued, the design cannot be changed. The certificate reflects a determination made by the regulating body that the aircraft is manufactured according to an approved design, and that the design ensures compliance with airworthiness requirements. The regulating body compares design documents and processes to determine if the design meets requirements established for the type of equipment. Requirements established by a regulating body typically refer to Minimum Operating Performance Standards (MOPS) and related documents (such as DO-178 series, DO-160 series and DO-254 series), which are developed jointly by RTCA, Inc. and EUROCAE. Once issued, the aircraft "type" meets appropriate requirements. The determination process includes a step called "First Article Inspection", for it and for each of its subassemblies. This is a quality control assessment whereas those prior to it are part of quality assurance.
With respect to "cannot be changed": When an airframer wants to change something it has two options. One is to request a Supplemental Type Certificate (STC), the other is to create an entirely different design. The choice is determined by considering whether or not the change constitutes a new design (i.e. introduces risk not considered in the first design). If the manufacturer believes the change doesn't introduce new risk the manufacturer typically requests an STC. This is less expensive. If the regulatory authority agrees with the rationale for choosing STC, the STC is granted