In SAP ERP, transaction codes starting with 'XK' (e.g., XK01/XK02/XK03) are used for centrally managing vendor masters, while 'MK' codes (e.g., MK01/MK02/MK03) are for purchasing data. XK03 and MK03 are standard display transaction codes for Vendor Master, whereas ZK codes are custom (Z-transactions).