What does this mean regarding my work being registered with BMI?

';In the event that the registration information you submit does not properly indicate that a work is based on a public domain source, we reserve the right, if at any time such work is found to have a public domain source, to allocate to such work a percentage of the normal logging credit or, in the case of a work having little or no original material, to give no logging credit to such work.';





Specifically the part about a public domain. What do they mean by ';public domain';?What does this mean regarding my work being registered with BMI?
For example,


if you wrote a tune and it turns out that tune is ';greensleaves'; for example, it is not copyright infringment, because it is not copyrighted, but you can't claim credit for it, as it's already out there, in the public domain.


i.e if your work is NOT original, but does NOT infrindge copyright.





However with any contract, get a legal person to look at it.