Ancillary Descriptions (AD) in the GCMD consists of brief descriptions of:

• Data Centers (AD-Dc) (Example: National Snow and Ice Data Center).
• Projects or Campaigns (AD-Pj) (Example: the Tropical Ocean Global
  Atmosphere project, TOGA).
• Platforms (AD-Pl) (formerly called Sources; example: the spacecraft
  description for the Terra satellite).
• Instruments (AD-I) (formerly called Sensors; example: the remote sensing
  instrument, AVHRR).

Ancillary Descriptions provide related background information but do not
directly describe the data sets. However, they are usually linked to data
sets. For assistance in writing ancillary information, you may visit the
Ancillary Description Writer's Guide and use the tools to create or modify a description.