Spelling and usage error
"Upmost" sounds similar to "utmost", and people may mistakenly think they are interchangeable; "upmost" refers to the highest position, while "utmost" means greatest or most extreme in extent.
"Utmost" is the correct word to use when referring to something as being of the greatest degree or most extreme. "Upmost" is a less common variant and usually meant to refer to the highest position in a physical sense, but it is rarely used, leading to confusion.
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