Vector Fields will accept multiple values in a single entry. Typically multiple value fields are set up as an Array of Elements, but this is only one method of using the data. Vectors is the other alternative.
While Arrays typically accomodate multiple sub-Elements, Vectors must be linked in the code by the programmer. For instance you could have one Vector listing multiple Resources for an Improvement to be capable of discovering, and another Vector listing chances of such discovery. This would require being careful to ensure that both Vectors are the same length (or the code can handle it if they are not), and the Modder must be cautious to ensure that the values are aligned properly. Due to this requirement for extra caution, Arrays tend to be the preferred method of such multi-Elements data.