What words are stored in `~/Library/Spelling/en`?

John Gruber recently tweeted on Mastodon: > One of my favorite features on the Mac [...] is adding new words to the system-wide spelling dictionary. The feature dates back to Mac OS X 10.0 and I'm pretty sure was in NeXTStep a decade before that. > > And, even better, the custom words you add to the dictionary are stored in a simple text file, one word per line, at: > > ~/Library/Group Containers/group.com.apple.AppleSpell/Library/Spelling/LocalDictionary https://mastodon.social/@gruber/112560468582323979 In my case, the file is actually at ~/Library/Spelling/LocalDictionary, because I'm crazy and run an ancient version of macOS (10.9 Mavericks). I don't _think_ this is relevant to the rest of my question. After taking a peek at LocalDictionary, something caught my attention. ~/Library/Spelling/ contains another text file, en, which like localdictionary contains one word per line. These words are different from the ones in localdictionary, but like locationdictionary they seem like things I probably typed over the past few years and wanted spellcheck to learn about. For example, it contains the name of the organization I work for, and some people I know with unusual first names. What is this en file, and how is it populated? Why do some words end up in en while others end up in LocalDictionary? (I assume that on modern macOS, the equivalent file is located in ~/Library/Group Containers/group.com.apple.AppleSpell/Library/Spelling/en)