[pbronez]

Per NATO's security guidance [0], NATO SECRET is the second-highest of ~5 classification categories It " is applied to information the unauthorized disclosure of which would cause serious damage to NATO."

In decreasing order of sensitivity, the categories are:

COSMIC TOP SECRET

NATO SECRET (NS)

NATO CONFIDENTIAL (NC)

NATO RESTRICTED (NR)

NATO UNCLASSIFIED (NU)

[0] https://www.act.nato.int/images/stories/structure/reserve/hq...

[sandworm101]

Those NATO levels are all just the start. Every NATO member runs many of their own layers. Over and above NATO come levels that are country-specific. In the US that means things like "five eyes" or US-only, which are technically secret (ie not TS) but are still above NATO. Remember that anything NATO is going to be seen by a large number of countries, including Turkey and Greece.