Northbeach...there are a variety of specs available to use for any battery and mfrs. tend to emphasize THE particular spec that is most important for a particular application. For example..car batteries must provide starting power so the 20 hour discharge rate is not very important though it could be figured out. Instead the mfrs. focus on CCA's or cold cranking amps. The construction of an auto battery is designed to maximize the short term amps avsailable. Such batteries are great for cars but in a boat will only last about 1/10th of the time as a deep cycle battery.
In a boat with deep cycle batteries...the longer term potential of the battery is more important so the mfrs. focus on the 20 hour discharge AMP hours and have defacto agreed on this as a means of comparing different batteries. Another important spec is the "# of cycles" which tends to be related to how much lead is in the battery. (Note...When comparing cycles between mfrs. make sure the depth of discharge is the same in counting a cycle.)
You can make a cheap battery with the SAME amp hour rating that will only last 1/2 as long as another with better construction. So...those are the 2 specs most commonly quoted on deep cycle batteries for marine AND other deep cycle use (golf cart batteries, RV house battries, fork lifts, trolling motors etc.) My "rule" applies to all deep cycle batteries of flooded, AGM or Gel "lead/acid" type...but you may find that such batteries in use for other industries use a different defacto standard to compare...commonly used are the 8 hour, 20 hour and 100 hour amp hour ratings. You may also see a RESERVE CAPACITY rating in minutes/hours which is a measurement of how long a battery will last under a 25 amp load before hitting 10.5 volts.
In theory, nicad, L-ions and other types specs should reflect the same type of specs but I cannot speak to that as my experience is primarily with boat systems and these type batteries are not presently in use in boats.