Send to Unreal uses NLA strips to associate particular actions with particular objects. Understanding this relationship is essential to exporting your actions correctly. This video gives you more information about it.
Here is the link to the full documentation:
https://github.com/EpicGames/BlenderTools/wiki/Send-to-Unreal-NLA-Strips
If you get a 404 error when you click the link above, then you will need to sign in to your GitHub account and link it with your Epic Games account. Here is a link to instructions on how to link your account:
https://www.unrealengine.com/en-US/ue4-on-github
source
