The notation "B_n" typically refers to the n-th Bell number, which counts the number of ways to partition a set of n elements into non-empty subsets. For example, the Bell number B_3 equals 5, indicating there are five different ways to group three distinct items. Bell numbers can be calculated using various methods, including Bell's triangle or recurrence relations. They have applications in combinatorics, computer science, and probability theory, helping to understand the structure of sets and the organization of data.