With multiple field configurations, Jira determines when to apply each of the configurations through the field configuration scheme. A field configuration scheme maps field configurations to issue types. This scheme can then be associated with one or more projects.
This allows you to group multiple field configurations mapped to issue types and apply them to a project in one go. The project will then be able to determine which field configuration to apply, based on the nature of the issue. For example, for a given project, you can have different field configurations for bugs and tasks.
This grouping of configurations into schemes also provides you with the option to reuse existing configurations without duplicating work, as each scheme can be reused and associated with multiple projects.