Show Ruler on Axis12 causes a ruler, or vertical numeric scale to appear to the right side of the locked axis. This feature tends to work best when Set Origin11 is set to the bounding box origin (otherwise the ruler is likely to interfere with the object itself).
The ruler scales from the maximum locked axis value (displayed at the top) to the minimum locked axis value (displayed at the bottom); the larger the viewing window, the more intermediate values the ruler can display. The nature of the ruler and the 3D projection means that sometimes the ruler may be partially obstructed by the object.
The ruler in this case scales from Z minimum (bottom) to Z maximum (top).