The difference between watch winder modes "B" and "L" (or sometimes "M") typically relates to their winding patterns and intended use. Mode B is designed for frequent wear, offering a shorter cycle of rotation (e.g., 2 minutes of winding followed by 6 minutes of rest) and repeating this pattern continuously. Mode L (or M) is typically for long-term storage, featuring a longer winding cycle (e.g., 10 minutes in each direction for 3 hours, followed by a 9-hour rest) and repeating this longer cycle.

Here's a more detailed: 
Mode B (or Frequent Wear Mode):
This mode provides a more active winding pattern, suitable for watches that are worn regularly. The shorter intervals of winding and rest help keep the watch wound and ready to wear without over-winding it.
Mode L (or Long-Term Storage Mode):
This mode is designed for watches that are not worn frequently. The longer winding cycle and rest periods ensure the watch is kept wound, but with less frequent activity, which can be gentler on the movement when the watch is not being worn.
In essence, the difference is in the duration and frequency of winding and resting cycles. Mode B is for active wear, and Mode L is for storage, ensuring the watch is kept wound and ready for use, even when not being worn regularly.