The backstory is that this is from some inherited code that tries to guess what diet might generate a particular isotope balance measured in bones from an archeological dig. The current technique identifies seven food groups that alter the isotope balance of three elements in different ways. There is not enough information to determine the diet directly from the isotope information so some other technique is required.

The technique adopted by the software I'm working on generates random diets comprising some proportion of each dietry element. It then calculates the expected isotope balance each diet would generate and records information for those that generate a balance close to the target balance.

The search space can be narrowed somewhat if you can provide constraints on the proportion of the diet that can be contributed by each component. For example, it is not possible to live on a diet of shell fish alone so a limit can be placed on the maximum contribution that shell fish could make to the diet without causing death in short order (about as long as eating nothing at all actually!).

So the constraints represent the allowable range for the proportion each dietry element may contribute to the total diet. Obviously for any particular diet the total of the contributing components is 100%.

A simple set of constraints would be to set the mean for each element to 50 and the range to +- 50 - that is, allow any value. In that case the sum of the means would be 350. Completely legitimate in the context of the problem space, but it doesn't constrain the search much!