When a drink calls for this ingredient, it typically means Aperol – the sweetest, lowest-proof of the red Italian bitter family. While Aperol can often be used interchangeably with its bitterer cousins (Campari, Gran Classico, et. al.), it has a unique character that comes from its lower alcohol content and greater focus on sweet orange peels, along with rhubarb, chinchona, and a bit of gentian.
Cappalletti is also quite sweet and can work in drinks that call for Aperol, although it’s significantly more bitter.