The PFS kits (Precision Fitment System) provided with every BBS wheel are to insure exact and precise fitments for a particular vehicle/wheel application. We use machined billet aluminum rings that are machined to the exact hub diameter as the vehicle they are to be used for.
These PFS kits have been used by BBS since 1996 without a single issue.  These are not cheap plastic rings, as some companies use, nor are they cheap Chinese hub rings with sloppy tolerances. Our PFS rings go into the back of the wheel and are then secured with a lock-ring, ultimately making them a part of the wheel and making every wheel, then, Hubcentric to their respective application. These rings and the use of these rings is 100% safe and is approved for use by the TUV  (Technischer Überwachungsverein – in English – Technical Inspection Association) of Germany.
The TUV has some of the most strict and stringent safety guidelines of any country in the world. Wheels and wheel accessories NOT approved by the TUV are illegal and unable to be used on Germany roadways.  
*image for reference only*