According to the Inspector General's letter, that is the proper marking for the top level of classification found in Hillary Clinton's unsecured private email. Let's go through that, just to be sure we're all clear on exactly what she's done. All of this information below is available in open sources, but it is not well understood by (say) voters.
TOP SECRET is information whose release could cause "exceptionally grave damage to the national security." No one may access this information who has not been through the very thorough background investigation, and even then you must demonstrate need to know.
SI means "Special Intelligence," and is a subset of SCI, or "Sensitive Compartmentalized Information." This information is tightly controlled, so that not only do you need to have need to know, you must have been properly read into the specific program from which the information comes.
TK is "Talent Keyhole," which governs our best aerial and satellite reconnaissance. It is always SCI information, and is extremely sensitive because it gives enemies a sense of exactly how good our reconnaissance technology has become.
NOFORN means "not releasable to foreign nationals." This caveat is discouraged because "NOFORN" means not the British, not the Canadians, not the Australians, not New Zealand. You can mark the data to be shared with the other Anglosphere powers, our very closest allies, with the caveat "FIVE EYES," or "FVEY". We have a treaty with them that governs the controls of sensitive signals intelligence. If the Inspector General has determined this item was properly marked NOFORN, it means that the information was so sensitive that we shouldn't share it with the British or the Australians in spite of that treaty.
It sounds like the two emails must have included intercepts of the most sensitive sort -- too sensitive to tell our closest allies about -- and possibly satellite imagery as a file attachment (or at least detailed descriptions of same). No one could have mistaken either of those things for unclassified information.