A Pod should make available the full library by default. Users can fine tune their dependencies, and exclude unneeded subspecs, once their requirements are known. Therefore, this attribute is rarely needed. It is intended to be used to select a default if there are ‘sub-specifications’ which provide alternative incompatible implementations, or to exclude modules rarely needed (especially if they trigger dependencies on other libraries).
When specifying a pod as a dependency you cannot selectively include subspecs and only the default is included.
From the CocoaPods default_subspec documentation.