What is an ancient lead seal?

[autographcollector11]

I have heard about the use of ancient lead seals in Rome. From what I understand, they contained a hole in the back and which cord or string could be used. I have one from the third century A.D. however I am still not certain as to how they were used. Can someone explain to me that use and function of a lead seal? How do these differ from a bulla?
thanks

 

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