Public Domain

What is the public domain?

This is a rather complicated legal question, that I do not feel qualified to answer. Instead I refer you to the explanation provided by the Open Source Initiative

Shall I put material for BOLTS into the public domain?

No, if you created material and want it included in BOLTS, I recommend to put it under a explicit license. If you do not want to keep any rights on the material, Creative Commons Zero is a good choice.