The phrase is for referring to a noun "company" such as in the sentence: "I bought the upper mid-tier company". (Meaning a company that is middle tier but slightly higher and not quite top-tier). I am questioning if it is correct to use "mid-tier" as a compound adjective with a hyphen.
"Mid-tier" seems to be commonly used as a word and referred to in dictionaries with hyphens, but it is occasionally spelled "mid tier" in online publications and style guides seem to support that too. I've looked at style guides on using hyphens which say if a word after "mid" is not capitalized or a number it should be joined without a hyphen, such as "midsummer". So which is correct? And how does "upper" act as a part of speech? Is it an adverb modifying the "mid" or "mid-tier"?