Source

File: wp-includes/rss.php

function normalize () {
		// if atom populate rss fields
		if ( $this->is_atom() ) {
			$this->channel['descripton'] = $this->channel['tagline'];
			for ( $i = 0; $i < count($this->items); $i++) {
				$item = $this->items[$i];
				if ( isset($item['summary']) )
					$item['description'] = $item['summary'];
				if ( isset($item['atom_content']))
					$item['content']['encoded'] = $item['atom_content'];

				$this->items[$i] = $item;
			}
		}
		elseif ( $this->is_rss() ) {
			$this->channel['tagline'] = $this->channel['description'];
			for ( $i = 0; $i < count($this->items); $i++) {
				$item = $this->items[$i];
				if ( isset($item['description']))
					$item['summary'] = $item['description'];
				if ( isset($item['content']['encoded'] ) )
					$item['atom_content'] = $item['content']['encoded'];

				$this->items[$i] = $item;
			}
		}
	}