From the Expression Engine user docs:
http://ellislab.com/expressionengine/user-guide/modules/channel/relationships.html#id12
Since an entry can have multiple parent entries in multiple channels, potentially with the same field, we may need to specify which channel we want to examine for parents. To this, use the channel parameter:
I do not understand how it is possible to have the same field name across multiple channels? What am I missing in this statement? What is the purpose of the channel field in the parents tag, then?