From the
FAA Regulatory and Guidance Library:
A Special Condition is a rulemaking action that is specific to an aircraft make and often concerns the use of new technology that the Code of Federal Regulations do not yet address. Special Conditions are an integral part of the Certification Basis and give the manufacturer permission to build the aircraft, engine or propeller with additional capabilities not referred to in the regulations.
Special conditions are often a precursor to later general rulemaking proposals. Although originally specific to an airplane make, other manufacturers in a similar situation may elect to use them for their projects.
Interpretative material is published by the regulatory authority to illustrate one or several ways to meet a requirement that have been found acceptable in past certifications, for example FAA Advisory Circulars or EASA ACJ/AMJ material.