Firstly: Sorry for answering the question with a placeholder

Actually, none. The Iberian Union was caused by a succession crisis in the House of Aviz because of the disappearance of King Sebastian of Portugal during the Battle of Alcácer-Quibir, in Morroco.

As such, the King of Spain became the heir of Portugal, resulting in one monarch ruling over both Spain and Portugal, which kept their administrative peculiarities. One thing we have to remember about this period is that an Union of subjects under the same monarch doesn't equate to the incorporation of one country into another.

Historian John Elliott argues that Europe before in this time period was comprised by what he calls "composite states". This means that instead of being a unified juridical and administrative entity, countries like Spain and France were a patchwork (more or less centralized) of different jurisdictions and administrative structures.

Of course, Portugal was an exception. And it was that way because it grew from a County to a Kingdom by conquering land from Muslims, whose legal structures were erased in the process. But when the Iberian Union came to be, this process didn't happen to Portugal.

This is the reason why kings of this time have so many titles. Phillip II of Spain/ I of Portugal, who ruled the peninsula under the Iberian Union was simultaneously king of Spain, Portugal, Naples, Sicily, Sardinia and Algarve. This isn't just for cosmetics, because these titles are how the king interacted with the local nobles and peasant elite.

This doesn't mean that composite monarchies are "aristocratic democracies", if this term makes sense, but it allows us to grasp why Spain has had problems with Catalonia for so long and why the administration of the colonies and the integrity of the Portuguese Colonial Empire weren't jeopardized by the Iberian Union.

TL;DR: Portugal and Spain were two distinct legal entities, but now under the command of one single king.

Source: ELLIOT, John P. ELLIOTT, John P. “A Europe of composite monarchies”. In: España em Europa. Estudios de historia comparada. Valência: Universitat de València, 2002, pp. 65-93.

Others interesting historians on the subject of the legal fabric of Iberian monarchies are: António Manuel Hespanha and Xavier Gil Pujol