Having an Aptitude or Knowledge in something can increase or decreate the Effect of an outcome.

You can only increase the effect by one, even if multiple Aptitudes or characters would apply.

Once you have increased the effect, another player cannot use a Setup Action to increase the Effect, unless the assisting player also has something that increases their Effect.

You generally start with two Aptitudes. Talk to the GM about Knowledge you want to start with.

Aptitude

Aptitudes are for general use with any Action, they are specific objects or types or things that you are very skilled at doing that come up a lot in the game. You have to pick them and pay for them at  character creation or leveling up, you only have a certain amount.

These can be used to increase the Effect of an Outcome that they logically apply to.

If you have an idea of one that isn't listed, talk to the GM. For example, if the game has people Forging Art a lot, that might be a reasonable aptitude to add.

Aptitude in a Guise

Instead of picking one of the Aptitudes, you can instead create a guise or 'character' for Grifting. It can be a fully fleshed-out alias, or a stock characterization like 'stoic bodyguard' or 'hippie' or 'nerdy scientist'.

You can change these during downtime if you find yourself not using the ones you have.

 Knowledge

Knowledges only apply to gathering information. It is a topic that you have information about. You have a broad background in that topic, so you are not starting from normal basic knowledge that everyone is assume to have.

You cannot use Knowledge for other Actions. Sometimes with Knowledge, the GM might just choose to not require a roll for gathering information, or just give the information that a standard Effect success would have given. The character doesn't even need to gather it, because they already know it.

Unlike Aptitudes, you can't level up Knowledge and also don't have any limit. The GM is encouraged to just let players have Knowledge that seem interesting and approriate to their backstory, or make up new ones. Possibly no one even realized it should be written down as 'Knowledge' until you say 'Wait, in my character backstory, I spent years stealing cars, surely I'm good at identifying cars from video', the GM agrees, and now you have 'Car identification' as a knowledge.

You can even treat past episodes in this way. If the team spend a bunch of time dealing with a specific obscure industry, and happen to run across that again a year later...that would certainly be useable as existing Knowledge if they needed information. However, at some point there's not really any purpose at writing it down, because the base knowledge level of all the charcater is just higher and the GM should just know that.