Like M.U.L.E., the game is about four colonists (players) building up an economy on the planet Irata (Atari spelled backwards). The objective is to become the richest colonist within twelve game turns, while also attempting to improve the overall health of the colony. In contrast to M.U.L.E., the setting is transferred to a sub-oceanic world. The most notable change in game play is the addition of a fifth choice of how to outfit the labour element. The new type allows the player to build his own labour elements (M.U.L.E.s, or in this case, turtles), so that the player won't have to buy them from the store.
Subtrade was developed in 1993 because the platforms that could run the original M.U.L.E. (mainly the Atari 400/800 and the Commodore 64) were slowly disappearing. Unlike today, there were no convenient emulators available to run old software on current machines. Due to the rising demand of players for a port of M.U.L.E. to newer systems and Electronic Art's refusal to develop it, several companies decided to develop unlicensed M.U.L.E. clones on their own. Besides Traders, Subtrade is the only M.U.L.E.-clone that has been commercially released for 16-bit home-computers.
Source: Wikipedia, "Subtrade," available under the CC-BY-SA License.