Cross-Localization is the App Store behavior where a single country storefront indexes keywords from more than one localization — letting you rank for extra terms by filling secondary localizations users in that country never see.
Every App Store storefront has one primary (default) localization, but Apple also crawls and indexes the keyword fields of certain secondary localizations in that same storefront. The United States, for example, indexes English (U.S.) plus around nine other localizations such as Spanish (Mexico) and French. Each indexed localization gives you another 100-character keyword field that ranks in that country — so a U.S. listing can carry roughly ten times the keyword coverage of an app that only fills English.
The catch is that phrases only combine within a single localization. The App Store builds multi-word search phrases from words in the same metadata field, so secondary localizations are best used for additional individual keywords you haven't already covered, not for rebuilding the same phrases. Used well, cross-localization is the closest thing ASO has to free keyword inventory: more terms ranked, with no change to what most users in that market actually read.
Example
A budget-tracking app targeting the U.S. fills its Spanish (Mexico) keyword field with English fintech terms it ran out of room for in the primary field — those words now rank in the U.S. storefront even though the visible listing stays in English.