Optional Text. Specifies an alternate attribute in the document processing instruction, specified by the PrimaryPITarget, ...

Optional Text. Specifies an alternate attribute in the document processing instruction, specified by the PrimaryPITarget, to use as the column value.
If you specify the PrimaryPIAttribute attribute, you must also specify the PrimaryPITarget attribute. If specified, SharePoint Foundation uses these values before those specified in the PITarget and PIAttribute attributes.
Used for property promotion and demotion in XML documents.