The term functions primarily as a proper noun. As the specific, registered name of a multinational corporation, it designates a unique entity and is therefore capitalized.
In syntactical analysis, while its lexical category is a noun, the term is frequently employed as a noun adjunct (or attributive noun). In this capacity, it modifies another noun to specify brand or origin, as seen in phrases like "a Xiaomi smartphone" or "the Xiaomi ecosystem." In these constructions, it performs an adjectival function by describing the noun that follows it, although its grammatical classification remains that of a noun.
Consequently, its grammatical role is determined by its function within a sentence. It is identified as a proper noun when referring to the company itself but acts as a noun adjunct when used to qualify another object. This dual usage is a common linguistic feature of brand names, allowing them to serve as both specific identifiers and descriptive modifiers.
