Small tubes fall under three main rating systems, well described in the GE Essential Characteristics manual:

Design-Center Rating System: To establish conformance to the design-center rating system, the ratings should not be exceeded with a bogey tube operating in the equipment under average conditions with respect to supply voltage, signal, temperature, component values, adjustment of controls, and other variables.
Design-Maximum Rating System: To establish conformance to the design-maximum rating system, the ratings should not be exceeded with a bogey tube operating in the equipment under the worst probable combination of conditions with respect to supply voltage, signal, temperature, component values, adjustment of controls, and other variables.
Absolute-Maximum Rating System: To establish conformance to the absolute-maximum rating system, the ratings should not be exceeded with any tube of the specified type operating in the equipment under average conditions with respect to supply voltage, signal, temperature, component values, adjustment of controls, and other variables.
The term "worst-probable combination of conditions" used above is not intended to include conditions under which useful performance of the equipment could not be obtained, since the equipment is not likely to be operated for long under such conditions. (3)

      A few notes on the above definitions: A bogey tube is one that exhibits average characteristics, i.e. all parameters fall in the middle of their "bell-curve" distributions. The "absolute-maximum rating system" defined here is not the same as for semiconductors; note that the absolute maximum ratings can be exceeded, as long as this doesn't last long.

      The absolute-maximum rating system came from the earliest days of radio when battery power supplies were used. Supply voltages could fall from the spec'ed value, but could never go above it. When used with AC or automobile supplies, it was the responsibility of the designer to account for all known variations, and keep the tube within its ratings. This system was used until the end of tube manufacturing in the 1980s for American industrial and military tubes and for all tubes in Europe.
American radio designers started using the absolute-rating specs as nominal maximums, i.e. if 115V on the power line gave a plate voltage of 135 volts, they would use this on an 01A (whose absolute maximum plate voltage rating is 135 volts), without considering that the line voltage could vary by at least +/- 10%. Components that affected the maximums typically had tolerances of 10 to 20%, further increasing the possible worst-case voltage. The result was a lack of reliability due to tube ratings often being exceeded.
       The design-center rating system was created by the RMA (Radio Manufacturers Association) to get around this problem. It was used on American consumer receiving tubes released from 1939 to 1957. This system is easiest on the designer - he does not have to worry about all the worst case conditions - he just designs for the "design-center". The expected variation in supply voltage, component values, and tube variations are factored in so that the real absolute maximums would not be exceeded. This results in fairly conservative ratings.
      In 1957, the RMA came up with a new system that put a bit more design responsibility back onto the engineer. The design-maximum rating system is the same as the design-center system, except that all variations in the tube's operating environment (line voltage, component values, etc.) have to be accounted for by the engineer. Variations in tube characteristics were still imbedded in this system, though. This change likely came as equipment designers were