What it says. The POD is early 1521. IOTL, she married John III of Portugal.
As a side note, why did the Habsburg siblings marry late in life?
For Catherine of Habsburg, the Alternate husband would be obviously the King of France or Duke of Bourbon.
Not Francis, Claude's still alive.
Also, would Charles really settle on a mere vassal Duke for his youngest sister? After all, Eleanor's sweetheart was the (future) Elector Palatinate, but he still married her off to Manuel.
What about Henry VIII, he is trying to annul his marriage to Catherine of Aragon already, we could also have Bloody Mary marry Charles V in this double marriage.
Catherine of Habsburg would give Henry VIII a couple of sons I think, in this time there is more time for her to bear sons for him.
Forgive my tardiness, but what of James V of Scotland? OTL he threatened the French with other marriages various times in order to get the French bride promised to him when it seemed as though the French were going to stiff him.
AFAIK he went after Catalina/Katharina's older sister, the Dowager Queen of Hungary, as well as her niece, the famed duchess of Milan. So why not Catalina/Katharina?