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I think a lot of the time people use "heirloom" to mean it's open-pollinated (i.e. a stabilised variety that is not an F1 hybrid).
I don't know what open-pollinated, stabilised, or F1 hybrid mean but I would love it if you wanted to explain because it sounds pretty cool
Don't have time but this link gives a brief overview of open-pollinated, heirloom and hybrid: https://blog.seedsavers.org/blog/open-pollinated-heirloom-and-hybrid-seeds
A stabilised variety basically means a hybrid that has been grown out for several generations and selected for stable traits, so that it will breed true from saved seed again. (With some plants, it's still necessary to isolate them from crossing with other varieties to make this happen.)