In the FAA's ICAO flight plan documentation, it states the following:
Cruising level (maximum 5 characters)
ENTER the planned cruising level for the first or the whole portion of the route to be flown, in terms of:
Flight level, expressed as F followed by 3 figures (for example, F085; F330), or
*Standard Metric Level in tens of meters, expressed as S followed by 4 figures (for example, S1130), or
Altitude in hundreds of feet, expressed as A followed by 3 figures (for example, A045; A100), or
Altitude in tens of meters, expressed as M followed by 4 figures (for example, M0840), or
for uncontrolled VFR flights, the letters VFR.
Flight Level and Standard Metric Level are synonymous terms for the same value. Flight Level is measured in imperial feet. Standard Metric Level is measured in metric meters. Either, whether represented in feet or meters, is the distance at which an aircraft flies above the atmospheric level of standard pressure (Pressure Altitude). This is the Indicated Altitude shown on your altimeter when the Kollsman window is set to 29.92 inches of mercury (or the metric equivalent). In the US, Flight Level is what you would use at and above FL180.
Altitude, whether represented in feet or meters, is the distance at which an aircraft flies above the standard datum line of the average level of the sea (Mean Sea Level). This is the Indicated Altitude shown on your altimeter when the Kollsman window is set to the local atmospheric pressure (if it is accurate). In the US, altitude is what you would use below 18,000 feet MSL.
Depending on atmospheric pressure, there will be a transition space between 18,000 feet MSL and FL180.