system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is the CIA allowed to black out parts of declassified documents?

Initially I thought it was just to protect the names of specific people involved - and indeed, that is definitely one reason they do this - but sometimes in declassified documents you'll see entire sentences or even paragraphs blacked out completely. I know it's the government, not just the CIA, who decides what gets declassified, but what's the point of declassifying a document if the CIA first gets to cover up the most sensitive bits? In the case of particularly damning documents, such the recent torture pieces, what's stopping them from simply blacking out the entire document? Do we know anything about the type of information that normally gets covered up? Is there *any* policy on this, or does the CIA just kind of do it and expect people to deal with it? Is it even carried out by the CIA in the first place, or is somebody else in charge of this?

I've been wondering this ever since I saw an old document about Charlie Chaplin in the communism witch hunts, where nearly full pages of the document had been removed despite it being a supposedly well-known and well-publicized scenario. It seems more relevant now than ever, though, as the torture documents have similar censorship and this is extremely unnerving for such an already controversial release.

EDIT: Thanks, everyone. The answers explained the law in place to determine what should and shouldn't be blacked out, and shed some light on the process and who's responsible for it. A lot of the info on what actually gets cut is still fuzzy, but that seems natural considering nobody can actually know exactly what that info used to be.

EDIT 2: Even the information on what's actually cut seems pretty clear now, or at least the type of thing they normally remove. I marked this explained a while ago but the explanations that are still coming in have been very helpful too. Thank you.
gpt
The easiest, non-conspiracy way to explain it is simply that the act of removing the information is what allows the documents to become declassified.

For example, in the sentence:

"Mr. Ahmed Jones informed us of the super secret password to the al-qaeda clubhouse: pepperoni."

The sentence could be "declassified" by removing sources and methods:

"Mr. [redacted] informed us of the super-secret password to the [redacted]: [redacted]."

When you remove the sensitive information, there's nothing left to warrant the classification, so it can be downgraded.