What is a Facebook Timeline Unit

https://developers.facebook.com/docs/changelog/

Facebook respond with this in their latest changelog:

*publish_action permissions will not be granted if there are no timeline units to be rendered. (rE451346)*

For the above reason I cannot request the publish_action permission in my facebook application. My question is what the heck is a t开发者_运维技巧imeline unit?

Cheers Rob

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Here is a detailed discussion about Open Graph Permissions and public actions permissions