A global query is a query that is defined for the whole site and made available for use in projects as needed.
A global query is defined for use with one or more global artifact types.
When you define a query using a single global artifact type it can be selected as available to be used in all projects that use that particular artifact type, in which case it shows up in the project as a saved global query.
When you define a query using more than one global artifact type:
In both cases a global query is available for addition to an existing project.
If you have the Domain Issue Tracking - Configure permission, then you can define a global query. If you have the or the Project Issue Tracking -Query permission, you can add the global query to your project.
As an example, when you define a global query, you may choose to include the artifact type Defect. The projects may use this particular artifact type or a set of artifact types that include Defect. All projects that use the "Defect" artifact type have commonly defined queries to show the "Defects discovered in the past 30 days" or "Defects assigned to a particular assignee."
These generic domain-defined queries can help with task management.