What do these pstart and pstop values mean? I'm attempting to tune a query and I want to be able to tell if partition pruning is happening. On one part of the query, the I'm getting a Pstart and Pstop value = "KEY":

| Id  | Operation                                 | Name                           | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |
|*  5 |      INDEX RANGE SCAN                     | IX_SOMEINDEX                   |     1 |       |     1   (0)| 00:00:01 |   KEY |   KEY |


Another line of the execution plan shows a Pstart and Pstop value = "ROWID":

| Id  | Operation                                 | Name                           | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |
|  10 |        TABLE ACCESS BY GLOBAL INDEX ROWID | LOOP                           |     1 |    18 |     2   (0)| 00:00:01 | ROWID | ROWID |


Basically I'm looking for definitions of what ROWID and KEY mean within the context of Pstart and Pstop.
Broadly, they mean that the partition(s) that need to be accessed will be determined at runtime.  In the first plan, where both are KEY, some earlier step in the plan is producing one or more values for the partition key.  Those partition keys are then used to determine which partitions Oracle needs to scan the index for.  In the second plan, the ROWID from the global index tells Oracle with partition to probe for each row that is being retrieved.