Which of the following concepts is associated specifically with SP-21?

SP-21, known as the Great Luo of the Spleen, is specifically associated with the concept of connection to all channels. This point acts as a significant junction that connects the Heart, Kidneys, and Stomach channels within the body. The Great Luo points are known for their role in facilitating the communication and flow between different channels, which is essential for maintaining overall balance and harmony within the body's energetic system.

The other concepts listed, while important in their own right, do not encapsulate the specific function of SP-21. For instance, major channel influence and regulation of Qi pertain to broader principles of meridian system operations but do not directly relate to the specific role of SP-21. Similarly, the confluent function typically refers to points that connect various channels and networks in a more generalized manner, whereas SP-21 serves a unique and vital role in the context of the spleen and its broader connections.