What do these pstart and pstop values mean?
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.