Where is "Unique ID (OpenID)" and how is it used?
OpenID was designed to provided a universal login to all your online accounts.  You can read more about it at http://openid.net/get-an-openid/what-is-openid/ .  It was created in 2005 around the same time as CiviCRM and I think the standalone version of CiviCRM would let you sign in using OpenID. There are modules/plugins that will let people sign into your CMS with OpenID.

As Karin has commented, if this is not a single step, one-time import of data, then you should use External ID to store the identifier from your other application.