What is your definition for rare pare

Something I am always interested in when rare pics are discussed, is how people define what a rare pair is and what not. Do you guys make it depend on the size of the fandom or not? Do you include pure smut fics into the consideration? Are there other things you also include in the consideration? For example: a ship has 4 fics with it. But the fandom only has 20 fics at all. Another ship has 100 fics but the fandom has 15.000. Which one would be a rare pic for you? Both, only the 4 one or only the 100 one? And another example a ship has 200 fics but 180 of them is the ship just beeing one of a dozen in Harem pics or it has one chapter in a one-shot-smut collection where every possible ship gets a chapter. Would that be a rare pic for you?