Jira allows only one person (assignee) to work on one issue at a time. This design ensures that an issue is a single unit of work that can be tracked against one person. However, in the real world, we often find ourselves in situations where we need to have multiple people working on the same issue. This may be caused by a poor breakdown of tasks or simply because of the nature of the task at hand. Whatever the reason, Jira provides a mechanism to address this problem through subtasks.

Subtasks are similar to issues in many ways, and as a matter of fact, they are a special kind of issue. They must have a parent issue, and their issue types are flagged as subtask issue types. You can say that all subtasks are issues, but not all issues are subtasks.

For every issue, you can have one or more subtasks that can be assigned and tracked separately from each other. Subtasks cannot have other subtasks. Jira only allows one level of subtask.