WordPress at a glance
filter is not described

pre_user_(field) filter-hook . WP 2.9.0

Filters the value of a user field in the 'db' context.

The dynamic portion of the hook name, $field, refers to the prefixed user field being filtered, such as 'user_login', 'user_email', 'first_name', etc.
*

Usage

add_filter( 'pre_user_(field)', 'filter_function_name_756' );
function filter_function_name_756( $value ){
	// filter...

	return $value;
}
$value(mixed)
Value of the prefixed user field.

Where the hook is called

sanitize_user_field()
pre_user_(field)
wp-includes/user.php 1225
$value = apply_filters( "pre_user_{$field}", $value );

Where the hook is used (in WP core)

Does not used.