Selecting an MOS

Hello,
I have wanted to become a Marine for many years and the time has finally come for me to enlist. I have talked to many Marines about enlisting and I have also talked to a Marine who has been in for over 20 years and at one point had recruiting duty. He told me that I can sign open and also sign a statement of understanding which garuntees me that I will be able to select a job of my choice when the jobs open up in october. He and my recruiter both have told me this as well as that I can be garunteed a specific field that I want, just not the specific job within that field. Is what both of them say true, or could I end up with another job that I don't want to have.