Chapters generally cover large areas (countries, states/provinces) and affiliates cover smaller geographic areas (cities, regions).
(Note, there are some early city-wide chapters that were grandfathered in, but ACBS now prefers groups that are mostly city-oriented to adopt the Affiliate model... but that is negotiable.)
Chapters are not "over" Affiliates; Affiliates do not "answer to" Chapters.
Chapters and Affiliates are encouraged to work together and combine their resources, but it is not required. A Chapter may choose to invite Affiliate leadership to participate in Chapter work/decisions (or vice versa), but it is not required for either group.
Affiliates are "affiliated" directly with ACBS, as are Chapters.