Confluence has been updated to version 6.15.9

Synchronization between two instances requires a configured Connection.

One side needs to initiate the connection and send an invitation to the partner (the Destination instance). 

The other side, needs to finish the configuration from their side. 

You can synchronize with different issue tracking systems. For more details check typical use cases

Start synchronization with your partner

To start a synchronization with your partner - Initiate Connection

Have an Invitation code? 

If you have an Invitation code - Accept an Invitation.