Final answer:
Xiu, a BCBA, reached the descriptive level of scientific understanding by determining the function of a client's behavior to access tangible items through an analog functional analysis.
Step-by-step explanation:
When Xiu, a Board Certified Behavior Analyst (BCBA), conducted an analog functional analysis and determined that the function of the client's challenging behavior was to access tangible items, she reached the level of description in the scientific understanding. Description involves observing and characterizing events or behaviors, and in this case, Xiu described the relationship between the challenging behavior and the outcomes that maintain it (access to tangible items). While the ultimate goal for many scientific inquiries might be controlling or predicting outcomes, simply being able to describe the function of a behavior is a foundational step in understanding.