WordPress at a glance
filter is not described

the_content_feed filter-hook . WP 2.9.0

Filters the post content for use in feeds.

Usage

add_filter( 'the_content_feed', 'filter_function_name_2525', 10, 2 );
function filter_function_name_2525( $content, $feed_type ){
	// filter...

	return $content;
}
$content(string)
The current post content.
$feed_type(string)
Type of feed. Possible values include 'rss2', 'atom'.
Default: 'rss2'

Where the hook is called

get_the_content_feed()
the_content_feed
wp-includes/feed.php 201
return apply_filters( 'the_content_feed', $content, $feed_type );

Where the hook is used (in WP core)

wp-includes/default-filters.php 206
add_filter( 'the_content_feed', 'wp_staticize_emoji' );
wp-includes/default-filters.php 207
add_filter( 'the_content_feed', '_oembed_filter_feed_content' );