High quality New Zealand sphagnum moss is the medium of choice for the vast majority of Neo growers, including those in Japan. Sphagnum offers high acidity, high humidity, and excellent air circulation around the roots. Sphagnum is an excellent growing medium for many orchids making it a versatile medium as well.
To use the moss, add water to the ENTIRE BLOCK to rehydrate the strands, and then gently pull apart the fibers. Rehydrate the whole block at once to avoid cutting fibers into shorter pieces. Unused portions can be stored in a closed container for later use–moist sphagnum kept in the dark will keep well for many months. Alternatively, you can allow the remainder to dry out to store it indefinitely.
In the past, the number of A’s indicates the length of the fibers: 3A works well for general potting, but for finishing Neo mounds, longer fibers found in 4A and 5A moss are very helpful. 4A and 5A moss must be harvested by hand to preserve the long fibers, thus the price. In fact, those long 5A strands are now so ridiculously expensive, almost nobody is carrying them. Now, the designations for strand length have changed to “Classic,” “Premium,” and “Supreme,” which are roughly equivalent to 3A, 4A, and 5A, respectively. These blocks are Classic or Premium (bags are unmarked, so I’m not sure).