Tech Range sets the maximum and minimum tech levels you should choose.
This spreads the Modifiers (from 0.5 to 2.0) between the two.

If you decide that all the players have a tech level choice between +3 to -3 then set the Tech Range to 3.
Tech level 3 will have the max modifier of 0.5 while Tech level -3 will have a modifier of 2.0

It spreads the max and min modifiers over the range chosen, so the higher the range the smaller the difference in modifier per Tech level.

In theory tech levels are available up to +10 / -10 but I cannot recall anyone asking for more than +5 /-5.

It is probably something I will look at in the shipyard.

Paul