If this returns TRUE then node is a deterministic node, which means that: given values for its parents, its value is determined with certainty.

There is no API function to directly set whether a node is deterministic, but setting all its conditional probabilities (i.e., CPT entries) to 0 or 1 will make a node deterministic. Building its table just with SetNodeFuncState_bn or SetNodeFuncReal_bn also will. Note that a node with a deterministic equation can result in a non-deterministic CPT, due to uncertainties introduced in the discretization process.