The spacing is such that when the vegetables are fully grown, their leaves just barely touch each other, creating a microclimate in which weed growth is suppressed and moisture is conserved. Raised beds produce a variety of benefits: they extend the planting season, they can reduce weeds if designed and planted properly, and they reduce the need to use poor native soil. Since the gardener does not walk on the raised beds, the soil is not compacted and the roots have an easier time growing.
A double skinned wall provides an air pocket of insulation that minimizes the temperature fluctuations and drying out of the soil in the garden bed. Sometimes raised bed gardens are covered with clear plastic to protect the crops from wind and strong rains.
Numbering starts in the upper left corner and moves left to right, top to bottom. For example, planting square A1 is the upper left-hand square (1st column, 1st row), while planting square C4 is the square in the 3rd column and the 4th row. I have yet to find a commercially available garden bed with an included feature for marking off planting squares.
They are perfect for the disabled gardener, those suffering from arthritis, and the elderly as they can be elevated to a suitable level so that bending over and kneeling will not be necessary.