Covers part of the difference between the actual healthcare costs and those covered by the Social Security. This is provided by private insurers with a variety of coverage levels/rates. It is compulsory coverage, with rates (fixed amount per month) and levels of coverage determined by law (minimum) and, sometimes, by collective bargaining agreements.
However, there are a few cases when the employee can refuse the coverage (e.g. if they are already covered by their spouse’s compulsory coverage and produce proof of this yearly, or if they are on a less than 12 mos. contract and don’t wish to benefit from the company’s contract etc.)
NB. France is considered to provide one of the best overall health care systems in the world, which is already covered by Social Security benefits. For example, if you are hospitalized or have a long-term ailment and are treated in one of the very good public hospitals or with a physician (widespread) who applies Social Security recommended rates, there will in fact be no cost left for a private healthcare plan to pick up.
This new benefit will mainly serve for the bits that are not so well reimbursed by Social Security (e.g. eye-wear / dental care / « everyday »/comfort medicine – e.g. for colds etc, for which reimbursement is not 100% – and extra costs above Social Security rates in private clinics, with physicians who go over SS rates)
This is why a private healthcare plan – despite being an additional cost – is not really expensive (usually under 50€/month).