It's not too clear in English. My best guess would be that the C40 is an organisation for the 40 largest cities, but that other smaller cities and towns are allowed to join, not as full members but as affiliate members.
Former US President Bill Clinton urged leaders of the world's cities, which produce over two-thirds of greenhouse gas emissions, to act swiftly to save the planet for their grandchildren. Skip related content Related photos / videos
Executives from the 40 largest cities plus 17 affiliate municipalities attended the C40 Large Cities Climate Summit which opened in Seoul, the third such event since 2005.
The former US leader, whose Clinton Climate Initiative develops programmes to help cities cut the emissions blamed for global warming, called for commitments and concrete action at the meeting which ends Thursday.