A total bottom is someone that only likes to get fucked. A submissive bottom is a bottom that is willing to let the top have control over him. How much control you are willing to give up depends on how submissive you are. A lot of people think dom-sub play means you're into doing kinky things and that is simply not true. You can be a dom or a sub without doing a lick of kinky stuff. Being submissive just means you are willing to give up your control to another person.
Also, just like how being dominant/submissive doesn't mean it has to involve kink, being submissive and being a bottom are also two separate concepts that don't always have to go together. Believe it or not, there are guys that are submissive tops. I've been with a few of them myself.
Being a submissive bottom is fun for the top if the top is into being dominant. It is fun for the bottom if the bottom is into it. It's a power exchange. One party enjoys having all the power, the other party enjoys giving up the power and letting another person have that control over him.
For me, I am a "total bottom" and but I can also be a "submissive bottom". I am, 99.99995% of the time, a bottom. But, some days I am also in the mood to be submissive. Some days I'm not. Whenever I do switch over to my "submissive bottom" mode, I become almost totally submissive.