Complex sentence: Public Domain Mark (PDM) is a symbol used to indicate that a work is free of known copyright restrictions and therefore in the public domain; it is analogous to the copyright symbol, commonly used to indicate that a work is copyrighted.
Split into simpler sentences: Public Domain Mark (PDM) is a symbol used to indicate that a work is free of known copyright restrictions and therefore in the public domain. It is analogous to the copyright symbol, which is commonly used to indicate as copyright notice that a work is copyrighted.