A component is a category of material type - or collection of matter - with defined characteristics which provides a useful grouping for use with all of Metsim's Component handling and transformation methods such as component separators and reactions.
Each component is different because of the difference in at least one property - or equation for a property - that provides a useful point of differentiation for your modelling requirements.
Metsim provides many of the "usual suspects" for characteristics, such as elemental composition, phase category, some physical characteristics and parameters describing the thermodynamic data. Using these you can do most of the manipulations you will require - such as heat and mass balances.
But the full potential of Metsim Components is unleashed when use your imagination and define specific components to suit particular modelling requirements. You could define different components for size intervals for rocks (using the component name) - but very quickly you realise you should just get the Particle Size, or Multiple Component Particle Size options.
For waste separation and processing facilities you may define component such as redPlasticBottles, cardboard, shreddedTyres, wasteOilDrums, and squashedCars - each of which has an average composition, average thermodynamic properties, and behave differently when presented to a separation or transformation process (such as hand sorting, screening, incineration etc.). The "Reactions" (or separation and recombining of constituents) for squashedCars would be different to the reactions for shreddedCars or wreckerStrippedCars as the processes result in different components (such as shatteredGlass or usedWindscreen).