TARGET = "_parent"
"_parent" is used in a situation where a frameset file is nested inside another frameset file. A link in one of the inner frameset documents that uses "_parent" will load new document where inner frameset file had been.
If current document's frameset file doesn't have any "parent", then "_parent" works exactly same as "_top": new document is loaded in the full window. Note that "_parent" doesn't work in a frameset, which is simply nested inside another frameset in same frameset file.