The compositions of generic alloys are specified in published standards. There, the compositions are presented as a set of rules. At most a% of element b, at least c% of element d, the sum of elements e and f must be between g and h%, and so on. For the MakeItFrom.com database, these rules are converted into mass percent ranges for each element.
It is common for minor alloying elements to be specified as a maximum with no minimum. In that case, the minimum is taken to be 0. Elements that are specified to fill the balance or remainder of the alloy get a specific, calculated range of possible values. Some standards (particularly those for steels) allow for specific optional changes to the alloy composition without altering the alloy designation. Those are not included in the database.
Since the composition value for each element is the full permitted range for that element, it is not necessary for the total of all minima or the total of all maxima (or any particular combination thereof) to add up to 100%. Instead, each element provides a separate requirement that an alloy must meet.
"Residuals" refers to the total of all alloy constituents other that those listed by name. Most composition specifications place a limit on the amount of residuals in the alloy. Some do not.