Usually there are two types of bonds, secured and unsecured. A secured bond requires money or a bond (promise to pay) be posted by a bailbonds man. An unsecured bond is what most people call "signing yourself out" of jail. In such case, there is still a bond sum or amount but the person can sign a promise to pay said sum in order to get released from jail. If they don't appear in court, a civil judgment can be obtained against them.
I have never heard of a "reference bond". Bonds can be either secured or unsecured, and a secured bond can sometimes be required to be a "cash bond", meaning that property or other securities cannot be used to post it, only U.S. currency. It is also possible to released on a "written promise to appear" which is exactly what it sounds like and does not involve any sort of security whatsoever.
No answer to these questions is intended to, nor does it, create an attorney-client relationship. There is no substitute for an actual conversation with a licensed attorney about the particular facts and circumstances of your case.