If an index has more than one key field, the precedence of the key fields is significant. For example, order lines have the key fields ORDER_NUMBER and LINE. The records can be indexed in two sequences: ORDER_NUMBER/LINE or LINE/ORDER_NUMBER.
DP4 asks you what you want the key sequence to be. Here, there is a natural precedence: ORDER_NUMBER comes before MATERIAL_CODE. Where there is no natural precedence, it does not matter which precedence you choose, but you have the option to control the choice. If you wish to use both indexes, you can do this by defining an index only table with the alternative key sequence.