From Flickr Commons statement:
"Participating institutions may have various reasons for determining that "no known copyright restrictions" exist, such as:
- The copyright is in the public domain because it has expired;
- The copyright was injected into the public domain for other reasons, such as failure to adhere to required formalities or conditions;
- The institution owns the copyright but is not interested in exercising control; or
- The institution has legal rights sufficient to authorize others to use the work without restrictions."