Here you go, section 21:
http://www.thefa.com/~/media/files/pdf/fa-competitions-1314/fa-cup/fa-cup-rules-2013-2014.ashx
'In all matches preceding the Semi-Finals, the Visiting Club shall have the right to claim up to 15% of all accommodation for which tickets are issued providing these tickets are in a fully segregated area, and subject to the following regulations: ' ...