It is not common for someone to refer to their romantic partner as both their "best friend" and "girlfriend" at the same time. These terms typically carry different connotations and are used to describe different types of relationships.
"Best friend" generally refers to a close, platonic friendship with someone who is highly trusted, valued, and with whom you share a deep emotional bond. It is a term often used to describe non-romantic relationships.
On the other hand, "girlfriend" is a term used to denote a romantic partner, someone with whom you are involved in a romantic and potentially exclusive relationship.
However, it's essential to recognize that relationships can be unique, and people may use terms in different ways based on their personal preferences and dynamics. In some cases, couples may describe each other as their best friend to emphasize the closeness of their bond within their romantic relationship.
Ultimately, the significance of the labels used depends on the individuals involved and their understanding of the terms. If you feel uncertain about the nature of your relationship with this person, it's best to have an open and honest conversation to clarify each other's feelings and expectations. Communication is key to understanding where you both stand and ensuring that you are on the same page regarding the nature of your relationship.