I know it is a huge storage for information the Cabal kept. But apparently I can also use it to erase myself from Imperial Intelligence records.

So, can the Black Codex just access and alter Imp Intelligence data, too? And if so, access any other data?

I really don't get this part.

I took it to mean the library of "back door" slices and access codes to the variety of computer systems that were under control by members of the Star Cabal throughout both factions. It probably also has logs on vulnerabilities of computer systems that were uncovered by people who worked for the Cabal, probably personal logs of members themselves. Maybe it even has dirt on dignitaries, officials, and politicians on both sides that you could use to blackmail to erase evidence of your existence.

It is also possible, given the Star Cabal's ties to SCORPIO, and her origins that we learn about in KOTFE/KOTET, and her time within the Belsavis prison, that it contains Iokathi and/or Rakata technology that has the capability to independently slice republic and imperial computer systems to wipe any evidence of you. The Rakata have an uncanny knack of being able to store people (or at least consciousnesses) in machines, and the Iokathi obviously had very sophisticated AI as well.

I took it to mean the library of "back door" slices and access codes to the variety of computer systems that were under control by members of the Star Cabal throughout both factions. It probably also has logs on vulnerabilities of computer systems that were uncovered by people who worked for the Cabal, probably personal logs of members themselves. Maybe it even has dirt on dignitaries, officials, and politicians on both sides that you could use to blackmail to erase evidence of your existence.

Checked over on Wookieepedia and that's basically what it says there.