Drupal 7 and before used one single package for core modules called 'Core'. In Drupal 8 there are so many different purpose modules and several logical groupings, that this did not make sense anymore. Also, we found that end users are looking for modules based on what they need not where the module came from. So we started introducing module groups in core as well.
Along these lines, modules primarily providing field types have been moved to the 'Field type' package. For example the text module that primarily provides an text field type uses package: Field type instead of package: Core in its info.yml file. Basically, if your module is doing other things and also defining a field, it goes in (another) package that makes the most sense for its functionality. If it only defines a field, it goes to this package.
