The necessary skills, abilities, knowledge, characteristics, interests etc. of workers differ substantially between occupations. Additionally, some occupations employ more people than others.
The Importance Weighted Demand of any given element is a measure that accounts for both of these observations. It is computed as follows:

For any occupation to which an element is relevant, we take the size of that occupation (as measured by the number of people it employs) and multiply it by a normalized measure of how important that occupation finds that element (as determined by surveys of people within that occupation). We do this for all occupations, then sum the results.