your license is for an upgrade version.
when starting your installation you're prompted for your xi serial number. when the installer fails to find a qualifying product (ie, previous version) installed, you will be presented with a dropdown of possible qualifying products. select the one you own. you will then be asked to enter the qualifying products serial number.
enter the correct number at the correct time and installation will proceed.
My previous version was Acrobat Pro 9 - which I deleted years ago and no longer have a serial number for. So what the heck do I do? Adobe apparently has no record of that purchase or its serial number.
If you lose the serial number you typically lose the use of the product.
you have 3 ways to recover a serial number that you once had:
1. if you purchased from or registered with adobe (automatically done with cs6):,
a. you can check your account, https://accounts.adobe.com
b. you can check with adobe support* (be prepared to give the email address used when registering purchasing)
2. if your adobe program is installed on another computer you can recover its serial number from that computer, (win:) http://www.belarc.com/free_download.html, (mac:) https://mac-product-key-finder.com/
using a browser that allows popups and cookies, contact adobe support during pst business hours by clicking here and, when available, click 'chat', http://helpx.adobe.com/x-productkb/global/service-ccm.html
Well, that's pretty silly - I had to supply the previous serial number to get the serial number for the upgrade to AP XI.
That's unusual. For many years Adobe have sold their upgrades as just open sales, and relied on serial checking on each install. Maybe you dealt with a reseller who did things differently. A big warning that you need to keep the serial would be a good thing, especially in the case where it finds the older product already installed (and hence, doesn't check the serial).
Thank you to kglad and Test Screen Name for the help - after hours of searching I finally found a file that had the AP 9 serial number so all is now good!
p.s. secure those numbers.