What causes this Server ID to be generated? Is it tied to the hardware, OS, or Confluence instance?

The Server ID:

• is generated when you install Confluence for the first time
• exists for the life of the Confluence instance
• survives an upgrade
• is held in the database
• is not bound to a specific licence
• is the same for all servers in a cluster.

What's the policy on re-associating licenses with server IDs?

There's no need to do this. Once you have a Server ID associated to a license, you can leave it as is.

What happens when I need to reinstall (quickly) on a different system?

Because the server ID is held in the database, it travels with the instance when the database or XML backup is restored on the new system. You need not generate a new Server ID for your new system.

What do I do when the license screen from my.atlassian.com is asking for my Server ID?

The Server ID is located on the license screen. If you have only a License ID, you may bypass the requirement to enter a Server ID - just look for the link from my.atlassian.com on my.atlassian.com after choosing "associate server ID." Check Unable to Find Server ID for Confluence 2.5.4 or Before for further info.

Additional Information

{htmlcomment}
http://support.atlassian.com/browse/CSP-17037
{htmlcomment}