Min tech level is just that, the tech level that must be reached before a ship can be built. It stops the AI from trying to build Cruisers when it doesn't have a Research Station, or Battleships when it doesn't have a ship upgrade station. It's a way of setting the speed that an AI advances technologically. The 5th number sets the max tech level that an AI will build an item. It's just another way to keep the AI from building a fleet of destroyers when it should be building battleships. So, if you want a Federation AI to build an unlimited number of Galaxies in tech level 3 but want it to stop building them at tech level 5, your 4th and 5th numbers should be -2-4, meaning that they will be built from level 3 up to and including level 4. As the BBOM says, consider "-3-4" to mean "3 to 4".