Built-in fields, such as priority and resolution, are global across JIRA. What this means is that these fields will have the same set of selections for all projects. Custom fields, on the other hand, are a lot more flexible.
Custom field types, such as select list and radio buttons, can have different sets of options for different projects or different issue types within the same project. This is achieved through what is called a custom field context.
A custom field context is made up of a combination of applicable projects and applicable issue types. When you are working with an issue, JIRA will check the project and issue type of the current issue to determine if there is a specific context that matches the combination. If one is found, JIRA will load the custom field with any specific settings such as selection options. However, if no context is found, the custom field will not be loaded.