Source

File: wp-includes/rest-api/class-wp-rest-request.php

public function set_param( $key, $value ) {
		$order     = $this->get_parameter_order();
		$found_key = false;

		foreach ( $order as $type ) {
			if ( 'defaults' !== $type && is_array( $this->params[ $type ] ) && array_key_exists( $key, $this->params[ $type ] ) ) {
				$this->params[ $type ][ $key ] = $value;
				$found_key                     = true;
			}
		}

		if ( ! $found_key ) {
			$this->params[ $order[0] ][ $key ] = $value;
		}
	}