The TREE Procedure
PARENT Statement
PARENT variable ;
The PARENT statement specifies a character or numeric variable that identifies the node in the tree that is the parent of
each observation. The PARENT statement variable must have the same formatted length as the NAME statement variable. If the
PARENT statement is omitted, PROC TREE uses the variable _PARENT_. If the _PARENT_ variable is not found in the data set, PROC TREE issues an error message and stops.