MPE_DERIVED is maintained through the EDIT screen, but LIBREF and DSN were free text - a typo in either silently produced a rule that never fired. LIBREF now offers the editable libraries and DSN the tables in the LIBREF on the same row, through the same SOFTSELECT_HOOK services the other configuration tables use (libraries_editable, tables_editable). VAR_NM stays typed: a derived variable names a column the table does not have yet, so a list of existing columns would be the wrong control. ACTIVE becomes the fixed pair 1 and 0 from MPE_SELECTBOX, with HARDSELECT so it cannot be typed freehand, and its NOTNULL rule carries '1' - the value the EDIT screen pre-fills on Add Row (getColumnDefault), because a derived variable only takes effect when active. The rows are written by mpe_makedata.sas for a fresh install and by the v7.19 migration for an upgrade, and each names the other so the pair cannot drift unnoticed; the ACTIVE guard itself lives once, in mpe_derived_check.sas. The mock hook carries the ACTIVE rule, as it carries every rule mpe_derived_check.sas enforces.