Why does the po_actions.close_po API default to the reason 'Close status rolled up'?

po_actions.close_po
(
p_docid => l_doc_id,
p_doctyp => l_doc_type,
p_docsubtyp => l_doc_subtype,
p_lineid => i.po_line_id,
p_shipid => i.line_location_id,
p_action => 'CLOSE',
p_reason => 'CLOSED BY WMS',
p_calling_mode => 'PO',
p_conc_flag => 'Y',
p_return_code => l_return_code,
p_auto_close => 'N',
p_action_date => SYSDATE,
p_origin_doc_id => NULL
)