A derived variable may need to reference one defined before it (a label that concatenates a derived region, for example), so the order has to be configured rather than assumed. - MPE_DERIVED: new VARIABLE_ORDER column (ddl, makedatamodel, migration). - mpe_derived: statements are generated in VARIABLE_ORDER then name sequence. VARIABLE_ORDER is required and must be a positive integer. A reference to a derived variable with an equal or higher order is rejected, because SAS would otherwise treat it as uninitialised and return a missing value with no error; references to lower-order variables are allowed. - mock: same ordering, each variable compiled against the physical columns plus those defined earlier, so the frontend can exercise a chained variable. - makedata: VARIABLE_ORDER on both demo rows plus a chained REGION_LABEL. - cypress: asserts the chained value in the grid. - tests: the getdata test seeds a second variable built on the first. - docs: VARIABLE_ORDER documented, with the ordered rule replacing the self-contained one.