I agree with your thoughts on this Ms. Lucie.
I was talking in the BDSM world not the slave trade where its not consentual. (however, that is sort of a fantasy of mine. one to remain as such, a fantasy)
here is my wording on this answer.
a submissive is someone who actually has most of the control in a BDSM scene because they set the limits, decide what will/will not happen, and can stop the scene at just about anytime they want.
a slave is the person who literally gives complete control to another person. this is generally something achieved over time and as trust builds between people. the slave will trust the Master/Mistress to respect limits even without expressing them directly. However, the slave is also open to streaching of the limits and expanding the experiances for the pleasure of the Master/Mistress. a slave will usually only have a few hard set limits and those are usually something like legal, damage, etc.
most relationships in the BDSM roles start with a Dom and submissive, but over time can turn into Master/Mistress and slave relationships. This change depends on the two parties involved.