top
Defines that all nets in the hierarchy will inherit the same wire load model as the top level block. Use it if when the plan is to flatten the design later for layout.
enclosed
Specifies all the nets (of the sub-blocks) inherit the wire load model of the block that completely encloses the sub-blocks. For example, if blocks X and Y are enclosed within block Z, then the blocks X and Y will inherit the wire load models defined for block Z.
segmented
Used when wires are crossing hierarchical boundaries. From the previous example, the sub-blocks X and Y will inherit the wire load models specific to them, while nets between sub-blocks X and Y(which are contained within Z) will inherit wire-load model specified for block Z
Not used often, as the wire load models are specific to the net segments