Unfortunetly Mr Boeing has taken out a lot of info from the manuals. Mine says:
OPT - displays the most economical altitude based on gross weight and the active cruise speed.
RECMD - displays the most economical altitude based on winds and airplane performance.
And from the flt crew trng manual:
Optimum alt is cruise alt for minimum cost when operating in the ECON mode, and for minimum fuel burn when in the LRC or pilot-selected speed modes. In ECON mode, optimum alt increases as either the airplane weight or cost index decreases. In LRC or selected speed modes, optimum altitude increases as either airplane weight or speed decreases. On each flight, optimum altitude continues to increase as weight decreases during the flight.
BF