The standard way in which values are stored inside Grasshopper parameters.

Data trees are collections of path and list pairs, where each list is a linear collection of values. I.e. multiple values can be stored together in a single, linearly indexed list, but not as more complicated structures such as grids or tables. The paths associated with all lists must be unique within the tree.

It is possible for trees to contain no values at all, a single value, empty lists, or any amount of null values. Furthermore each non-null value in a tree can be associated with any amount of meta data.