In the Security settings for an ArcGIS Online/Portal site there is an option called 'allow portal access'. I'm trying to understand exactly what this is for (and how to use it)? In ArcGIS Online if if add my Portal instance what does this mean? Can I now access AGOL content from Portal or does it work the other way around and I can access Portal content from AGOL? And how does this differ from creating and using a collaboration?