Bundles and the primary sat
A bundle sells several kinds of rare sat in one output; the site files it under one category, the rarest holding all its sats, else its primary sat's.
One category per sale
Every sale and every listing is filed under exactly one category, its own, so categories never count the same sale twice. "Rarest" means the category with the fewest sats that will ever exist.
When one category holds every tracked sat sold, the sale is the rarest such category's. A plain uncommon is an Uncommon sale, a 2012 uncommon an Alpha uncommon sale, a rare sat a Rare sale, a block 9 chunk a Block 9 sale and a chunk of block 9's first bitcoin a 450x sale, even when the chunk holds an alpha or a palindrome.
The primary-sat rule
A bundle is a sale or listing no single category holds whole, such as an uncommon sold with a 2D palindrome. It is filed under its primary category: the rarest category it sold sats of. On a tie in rarity, a first-sat category (the uncommon side: uncommon, rare, epic, alpha and the categories built on them) comes before a last-sat category (the black side: black, black rare, black epic, omega and theirs); categories of neither side sit between.
So the classic pair, the last sat of one bitcoin (a black omega) sold with the first sat of the next (an alpha uncommon), is an Alpha uncommon sale: both categories hold 628,419 sats.
The pair rule
One exception: an alpha uncommon (including the Kilosat to Megasat uncommons) sold with black sats is always the uncommon's sale, however rare the black category. A 2009 black (32,474 sats) is rarer than Alpha uncommon, yet a 2009 pair is an Alpha uncommon sale; Megasat black (20) is rarer than Megasat uncommon (21), yet the megasat pair is a Megasat uncommon sale. Only the black categories give way: a black rare or black epic, or a collectible omega such as a Nakamoto omega or Pizza omega, still leads by rarity, and a plain uncommon that is not an alpha does not take the lead from a black omega. Listings follow the same rule.
Secondary categories
Every other category a bundle sold sats of is a secondary category: the sale names them all, with how many of its sats each holds ("Sold as a pair: alpha uncommon (primary), holding a black omega, an uncommon and a black"). A Block 9 chunk holding a palindrome names its Block 9 palindrome and that sat's Nakamoto palindrome. A secondary category's median, volume and market never count the sale, but its collection's Activity lists it, marked "with" the category that led it and with no unit price of its own.
This is different from a sub-category: a sale made wholly of a rarer category that a broader one also holds, such as an alpha uncommon or a Pizza uncommon under Uncommon. The broader collection's Activity lists those too, marked with their category, and also outside its numbers. On the sales chart, Include rarer sub-categories and Include sales led by another category (both on unless switched off) do the same.
A bundle's unit price is per sat of its own category. A sale that splits sats out of an output counts only the sats it sold.