Comment by ameliaquining

First one flattens a potentially-nested tuple type. E.g., FlatArr<[number, [boolean, string]]> is [number, boolean, string].

Second one gets the element type of a potentially-nested array type. E.g., Flatten<number[][]> is number.

For what it's worth, I've never needed to use either of these, though I've occasionally had other uses for slightly fancy TypeScript type magic.