The Submesh settings can be found on the following pages of the Object Settings dialog:
When the scene is exported, a submesh is merged into its parent mesh. Make sure that scaling, rotation, or any modification of the parent mesh's pivot point is performed before adding any child meshes to be merged as submeshes.
Note that this limitation does not affect submeshes (meshes that have been made children of a parent mesh), which can be translated, scaled, rotated, and have their pivot modified at any time.