This statement deletes all orders in the orders table from partition 1 of the SLPPI table orders.
DELETE FROM orders WHERE orders.PARTITION=1;
If orders was a multilevel partitioned table, you might substitute PARTITION#L1 for PARTITION. However, this substitution would delete all orders in partition 1 of level 1 from the table, which is probably not the result you anticipated.
A DELETE statement for the same multilevel partitioned table specifying PARTITION in place of PARTITION#L1 would only delete the rows in combined partition 1.