hi there
i am sure that you mean "Contractors Health and Safety" being a scheme that deals with the pre qualification situation in the construction industry.
depending on what service you are offering, and what client you are dealing with, will determine whether they insist that all their suppliers are on the CHAS scheme or not.
the initial objective of the scheme, is to reduce paperwork, of each time you apply for a contract, by being CHAS approved, it reduces the need for a full PQQ.
Saying that a lot of developers are still using their own vetting system alongside CHAS.
regards
martin