BPMN21 — Page 317. Ambiguous associations “partitionElement” and “partitionElementRef”

Table 10.135.

a)
Model associations “partitionElement” and “partitionElementRef” have the same description:
“A reference to a BaseElement which specifies the partition value and partition type. Using this partition element a BPMN compliant tool can determine the FlowElements which have to be partitioned in this Lane.”

So, which is the difference between them?
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b)
Model association “flowNodeRefs” is described as “the list of FlowNodes partitioned into this Lane according to the partitionElement defined as part of the Lane element.”

“partitionElement” is optional. What happen if “partitionElement” is not defined?

Does “flowNodeRefs” depend of “partitionElementRef” also?

Can both “partitionElement” and “partitionElementRef” be defined at the same time?