Hop substitution is swapping one hop variety for another with a broadly similar alpha-acid range and aroma character, usually because the specified variety isn't available — a common, genuinely useful practice, but never a perfect one-to-one match, since the aroma-oil profile that actually defines a variety's character varies even among hops considered close substitutes for each other.
Matching alpha acid keeps bitterness on target
The bitterness side of a substitution is the easy part to get right — running the substitute variety's actual alpha percentage through the IBU calculator instead of assuming it matches the original keeps the recipe's IBU close to plan even when the specific variety changed.
Aroma is where a substitution genuinely changes the beer
Alpha acid alone doesn't capture what makes a hop variety distinctive — Citra's tropical-fruit intensity and Cascade's grapefruit-pine character sit in overlapping alpha ranges but taste nothing alike in a late-boil or dry-hop addition, so a substitution chosen purely by matching alpha percentage can hit the target bitterness while missing the intended aroma character entirely. For a bittering-only charge, a rough alpha-acid match is usually good enough; for a late or dry-hop addition where aroma is the whole point, matching the actual flavor description matters more than matching the number.