tatic UDFs (UDFs for short) are used to describe and explore relationships between the variables in your data such as (v1+v5+v9)/15. These UDFs are static in nature because the variables vi they use represent certain variables in your data and, in the parse trees, they always occupy a terminal or leaf position.
Dynamic UDFs (DDFs for short) behave exactly like a normal built-in function of GeneXproTools and therefore their variables or arguments are fluid, that is, their meaning will depend on their context on the parse trees. For instance, the DDF (e1+e2+e3)/15 will take three arguments and in the parse trees will appear as a bud node with three branches where each branch represents a particular expression ei. These expressions might be as simple as a constant or as complex as functions involving hundreds of variables.