MAST table — Material to BOM Link Table

MAST stores the assignment of a material to a bill of material. Each row says: for this material, in this plant, under this BOM usage, this alternative BOM number applies. The row itself carries no components and no quantities; it is purely a pointer from material master data into the BOM header table STKO, which then points down to the item table STPO. A single material can have multiple MAST rows if it is produced in several plants, or if it has several BOM usages (production, costing, engineering) or several alternatives within the same usage. BOM explosion logic in order creation, MRP, and costing all start by reading MAST to find the STLNR, then read STKO for header status and STPO for the item list.

MAST is client-dependent and keyed tightly, so selectivity is not usually the problem; getting the right combination of MATNR, WERKS, and STLAN is. Always restrict by plant before material alone, because the same material number can carry different alternative BOMs per plant with no relationship between them. Do not assume STLAN 1 (production) is the only usage in play; costing runs frequently read usage 1 or a costing-specific usage depending on configuration, and engineering changes often live under usage 2 until released. Check LKENZ before trusting a row — a deletion flag set here means the material-to-BOM link is inactive even though STKO and STPO underneath may still look perfectly valid.

Symptom: a production order picked up an unexpected alternative BOM. Select MAST where MATNR equals the order's material, WERKS equals the order's plant, and STLAN equals the usage the order type uses (commonly 1). List all STLAL values returned with their STLNR. Then check STKO for each STLNR for validity dates and status, and compare against the order's basic start date — the order takes the alternative whose STKO validity period covers that date and whose selection method matches, not necessarily the lowest STLAL number.

MAST is unchanged as a transparent table in S/4HANA and continues to be written by standard BOM maintenance transactions. Fiori apps for BOM management read through the same underlying structure, and CDS-based reporting for BOM data typically layers views on top of MAST, STKO, and STPO rather than replacing them. No structural change to MAST itself is expected across S/4HANA releases; the table keeps its role as the material-to-BOM pointer.

BOM assignment and alternative logic is owned by the PP or plant engineering master data team, usually the same group responsible for routing maintenance in task lists. Disputes over which alternative an order should have picked up get resolved by comparing MAST/STKO validity and selection settings against the order date, not by adjusting values in MAST directly.

STKO and STPO are the BOM header and item tables MAST points into; MAPL is the equivalent link table for task lists and is read alongside MAST during order and planned order creation. AFPO carries the order-level reference back to the BOM used. PLAF reflects planned orders that also resolve BOM assignment through MAST before conversion. Maintained through standard BOM transactions for creating, changing, and displaying material BOMs.