It is just the term; that VW uses to describe the plastic pipe that connects the head/block of the engine. They are plastic; so they get brittle and cracks are common. If you are concerned about them getting brittle; you can just replace them, while you are doing other work on the cooling system (they are not expensive). You could also; check your cooling system for leaks, by using a cooling system pressure tester.
The coolant tank; requires a special adapter, to be able to pressurize the system. You can buy adapters online, make your own with a cap (use your old one) or purchase a whole kit with the VW specific adapter included (harbor freight sells one that comes with the correct adapter). You will need; these types of tools, as a professional auto tech; so, purchasing now... will make you money later. The HF kit; is made out of plastic, may not hold up to a tough daily professional shop environment but for home use or careful shop use, you should be ok. We use this in my brothers repair shop; the pump ended up getting cracked but we still use the adapters that came in the kit and they work ok.
Radiator Pressure Tester Kit
Pittsburgh Automotive - Item#69258
Radiator Pressure Tester Kit
adapters for use with Stant style pressure tester and other options:
https://www.google.com/search?q=vw+..._sm=93&q=vw+coolant+pressure+adapter&tbm=shop
Classic Stant pressure tester:
http://www.stant.com/consumer-products/testers-adapters/cooling-system
https://www.google.com/search?q=Sta...m=93&ie=UTF-8#es_sm=93&q=Stant+12270&tbm=shop