I've never played Fate before and am learning from Fate Accelerated Edition with occasional reference to Fate Core. For the most part, it has been straightforward. On the other hand, I am somewhat unclear about how explicitly Situation Aspects must be declared.
For example, Criminal Charlie is pursued into a bustling marketplace and spends a few turns trying to outrun the authorities. Finally he decides to try to hide in plain sight and his player says, "I will spend a fate point to invoke the marketplace's Crowded aspect to make it easier to hide in the crowd."
Nobody has, at this point, put "Crowded" onto a index card on the table, but clearly the marketplace is a crowded place. It is not an aspect that must be created by changing the situation. It is not an aspect that is hidden and must be discovered. It seems like the easiest way to deal with this is to imagine that in a given situation there is a vast sea of implicit aspects, just waiting to be invoked. The GM gets to say whether any given one is there. Forcing the GM to write down every obvious aspect at the start of each scene seems unrealistic.
I found nothing in FAE to confirm or reject this idea, so I fell back to Fate Core. There is a bit of text on page 136 that may reject the notion that these aspects are "free":
[Create an Advantage] could also mean that you’re discovering new information that helps you (like learning the weakness of a monster through research), or taking advantage of something you’ve previously observed (like your opponent’s predisposition to a bad temper). [and following ¶]
I understand that Create an Advantage is about not only creating aspects, but also getting a free use. Does one have to Create an Advantage to take advantage of aspects that seem implicit, but were not yet declared?
Best Answer
Aspects determine what is important in the scene. Those scene aspects defined when the scene is framed are the ones that the GM deemed to be important in the scene; the ones that add flavor to the narrative.
However, narrative games like fate share the narrative direction with the players. In this particular case, though the GM didn't deem the fact that the marketplace was crowded one of the things that would add flavor to the characters' interaction with the scene, one of the players did. And because of that, and because he felt strongly enough about it, he took an action or spent part of his narrative capital (Fate points) to bring it into play.
As a GM, never feel that you have to define everything- only those things that are important should be defined before hand. And as a player, never think that you are confined by those things the GM has declared- Fate points and your skills give you the tools that you need to help to expand and drive the narrative.
To directly answer your final question, i.e. "I understand that Create an Advantage is about not only creating aspects, but also getting a free use. Does one have to Create an Advantage to take advantage of aspects that seem implicit, but were not yet declared?"
Creating an aspect is one way of getting the use of an implicit aspect. In this manner, you get a free invocation. However, if you're not looking for a free invocation, i.e. you just think that it's important to note that the marketplace is crowded, you can suggest it freely, and if everyone agrees, the Crowded aspect is added. This is stated in the Fate Core Rulebook on p. 78. It can also be found in the Fate Core SRD- Creating and Discovering New Aspects in Play.
Then later, if you, or someone else, wants to use it, a Fate point can be spent as normal to invoke it.