Use an existing standard extension type if you can; to do so facilitates use of existing analysis packages. New extension types are necessary only when information is organized in a way that existing extension types can't handle. Before starting to develop a new format, check with the FITS Support Office or IAUFWG to see if an existing or developing extension can be used to organize your data. Inquiries on sci.astro.fits (section 6.2) may also be helpful. If a new proposed extension type under development can be used, coordinate what you do with the developer. If a totally new extension is needed, try to design an extension that has as general an application as possible, with the eventual goal of adoption as a new standard extension. The type name of any new extension must be registered with the IAUFWG and be different from any already registered. The FITS Support Office maintains the current list. Special purpose extensions restricted to particular projects, with the exception of prototypes for more general designs, should be avoided.