Section 5.9 Querying the Layer number for any Port in the spec says:
There are times, especially during a cached setup, that this layering order value for a specific, not yet bound, port is important to know.
and wfdGetPipelineLayerOrder() is given as a way to retrieve the layer order.

Section Pipeline Layering says:
It should not be assumed that the layering order values are static, as they may change when the port to pipeline binding changes.
What can be done if the client app decides the layer order isn't appropriate?
There isn't a function to set the layer order. So, is the client app expected to move sources between layers to achieve the require layering?
If the layer order can be dynamic, how can the client app predict the order?
Is the order implied by the return order of wfdEnumeratePipelines or WFD_PORT_BINDABLE_PIPELINE_IDS?
Does section mean that the values specifying the order may change but the sequence may not?
Is the layer order based on source binding order?