[p2t2p]

I'll tell you a secret, there are no "subissues", there are only issue links and UI represents different kind of links differently ;-).

[Cogito]

Subtask issuetypes are a real thing, and are different to issue links.

Different parts of the UI and some functionality works differently for subtasks than for linked issues.

For example, you can set it up so that you can't close a 'user story' until all subtasks under it have been closed. Not possible out of the box with linked issues.

[paulddraper]

I'll tell you another secret: Epic Link is something else entirely.

[p2t2p]

Yep, it's a custom field, purely Agile's feature.