Release candidate software is a type of software that is just in the functional stage. It is tested within the laboratory and faces some glitches. These glitches should be addressed before taking the software to next stage.
Once glitches are addressed, the next stage for RC is to be tested by a selected number of consumers market, to which it is released. It is not released to broader consumer market at this stage.
Release candidate software does not take a shape of a product during the two testing phases and therefore is not marketed to all consumers. Once it passes the two steps, it is released as a complete product.
RC is a functional software but cannot be marketed at all during two phases. Consumers might know its features through information tipped to websites and know what type of software it will be, but they cannot use it.
Beta Version is also a term used for a functional software. But in case of Beta Version, the software has more glitches than RC. Both software products are functional and not marketable at this stage.
As it sounds, RC is a candidate for release in the near future, but it will have to clear all testing stages and take shape of a complete product before it is made available to consumers on the market.
If you are offered to download a type of RC software, do not do so. It can cause some serious risk to your system. The software might be a pirated copy of original version, which is yet to be released and it can damage your PC.
It is recommended to read information about a RC software. It is possible that it is not for use at all, and if it is, you can know about it before it is launched. However, if you are offered the software for testing purpose, you can use it and provide the feedback on the shortcomings or glitches.