Read metadata for attachment in BPM

 Hi

I need to read metadata for a WebCenter Content attachment in BPM process. The scenario is as follows

1. The BPM workflow is triggered with a incoming document (e.g. Purchase order)

2. We need to find the related document (e.g. purchase requisition) for this document. we search the repository from the BPM workflow

3. When the related document is found, the metadata for this document needs to be extracted and metadata for the current document needs to be updated (i.e. purchase requisition number needs to be added as reference number in purchase order metadata)

After searching, i found out that the ucmMetadataItem can be used for this, but there is no documentation for this. What is the way I can achieve the above?