Addressing a Daughter: This is a very common and loving way for parents or grandparents to refer to their daughter. “My `姑娘` is in college.” (我家姑娘上大学了。)
Addressing a Young Woman (by an older person): An older person might address a young woman they don't know as `姑娘` to be friendly and polite. For example, “Excuse me, young lady, you dropped this.” (姑娘,你的东西掉了。)
General Reference: It's used to talk about a young woman in a positive or neutral way. “She's a very capable young woman.” (她是个很能干的姑娘。)
Regional Use: The term is more prevalent in Northern China. In the South, especially in Cantonese-speaking areas, `靓女 (liàngnǚ)` meaning “pretty girl” is a much more common way to get a young woman's attention.
Formality: It's generally informal but polite. It's not typically used in formal business writing. In conversation, it strikes a balance between being respectful and friendly, avoiding the potential awkwardness of the term `小姐 (xiǎojiě)`.