1. Don't tell anyone about it. It's a top secret.
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If we want to talk about the next valuable secret, what expression can we use?
2. It's a second top secret.
3. It's the next top secret.
4. It's a second class secret.
(Do we have to use 'a' or 'the'? Which one is used? Do we have a suitable term for that?)
A secret is a secret. Government agencies may classify secrets, but they are all still secrets. We don't usually refer to a secret as having degrees or rankings.
When talking about the next valuable secret, you can use the expression "It's the next top secret."
In general, when referring to something specific, we use "the." So, in this case, since we are talking about the specific next valuable secret, we use "the" before "next."
As for the term to describe using "a" or "the," it is called an article. In English, there are two types of articles: definite (the) and indefinite (a/an). The definite article "the" is used to refer to something specific, while the indefinite article "a/an" is used to refer to something non-specific or if the noun is being mentioned for the first time.