Hop variety names get thrown around constantly in recipes and forum threads, but the number that actually determines how much bitterness a given weight contributes — alpha acid percentage — is rarely explained, and it isn't a fixed constant. It's a range, and it moves with the growing region, the season, and even the specific lot a merchant ships.
Every hop page here states that range as it's typically published by growers and merchants (Yakima Chief Hops, Hopsteiner, BarthHaas and similar sources), alongside the aroma character the variety is known for, whether it's typically used for bittering, aroma, or both, and a short list of varieties commonly substituted for it when the original isn't available. None of that substitutes for reading the Certificate of Analysis on the actual bag you're using — alpha acid on the label is what the IBU calculator should use, not the midpoint of a published range — but it's exactly what you need when planning a recipe or evaluating a substitution before you've bought anything.
The varieties are grouped loosely by origin — American, German and Continental European, British, New Zealand and Australian — because origin correlates strongly with character: German and Czech noble hops trend toward restrained spice and herbal notes at low alpha acid, American varieties trend toward high alpha acid with bold citrus and tropical fruit, and the newer Southern Hemisphere varieties (particularly from New Zealand) have carved out a reputation for intensely fruity, almost wine-like aromatics that don't map cleanly onto either older tradition.
If you're choosing a hop for a specific job — heavy bittering charge, late-boil flavor addition, dry hop — the "use" field on each page is the fastest filter: bittering varieties are bred and grown for a high, stable alpha acid content and a clean bitterness; aroma varieties trade some of that alpha acid for a more distinctive oil profile; dual-purpose varieties do a reasonable job at both. Once you've picked a candidate, the IBU calculator will tell you exactly what a given weight and boil time contributes, using both the Tinseth and Rager formulas.
Each variety page also lists a short set of substitutes — other varieties commonly reached for when the original isn't in stock. Substitution is never a perfect match; aroma character comes from a complex blend of essential oils and no two varieties, even close relatives bred from similar stock, share an identical oil profile. Treat the substitute list as the closest reasonable starting point, not a guarantee of an identical result.