WordPress at a glance
filter is not described

rest_query_var-(key) filter-hook . WP 4.7.0

Filters the query_vars used in get_items() for the constructed query.

The dynamic portion of the hook name, $key, refers to the query_var key.

Usage

add_filter( 'rest_query_var-(key)', 'filter_function_name_5920' );
function filter_function_name_5920( $value ){
	// filter...

	return $value;
}
$value(string)
The query_var value.

Where the hook is called

WP_REST_Posts_Controller::prepare_items_query()
WP_REST_Revisions_Controller::prepare_items_query()
wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php 889
$query_args[ $key ] = apply_filters( "rest_query_var-{$key}", $value ); // phpcs:ignore WordPress.NamingConventions.ValidHookName.UseUnderscores
wp-includes/rest-api/endpoints/class-wp-rest-revisions-controller.php 443
$query_args[ $key ] = apply_filters( "rest_query_var-{$key}", $value ); // phpcs:ignore WordPress.NamingConventions.ValidHookName.UseUnderscores

Where the hook is used (in WP core)

Does not used.