What is the difference between "Standard Metric Level" and "Altitude" in ICAO flight plans?
Standard Metric Level is the metric equivalent of the foot-based Flight Level. It is what you get when the altimeter is set to QNE, i.e. above the Transition Level, and read in meters.

Metric Altitude is the metric equivalent to the foot-based Altitude. It is what you get when the altimeter is set to QNH, i.e. below the Transition Altitude, and read in meters.

These are actually the official ICAO systems, with the foot-based systems declared a "temporary" alternative.