Development types are used to classify a grid cell according to the ``type'' of development currently in the grid cell. For instance, grid cells with only a few residential units and no other square footage might be classified as ``low density residential'' which may be abbreviated as ``R1''. Other grid cells may be classified as mixed use, commercial, etc. The set of development types to use is arbitrary.
Development types are grouped by two nested mechanisms: groups and non-overlapping-groups. Each development type may be a member of multiple groups. Each group may be a member of multiple non-overlapping-groups. All of the groups in a non-overlapping-group must be disjoint (i.e., may not share any development types); in other words, each development type must belong to at most one group in each non-overlapping-group.
Groups and non-overlapping-groups are used in the computation of the variables in the models, so to fully understand them requires understanding the model definitions.