Criteria for determining Directionality

Hello All,

I was inquiring about the criteria used to determine the DIRECTIONALITY field (incoming, originating, outgoing) of message tracking logs for Exchange Server 2013.  Unfortunately, the whitepaper (linked below) does not describe this. Any insight into how this is determined would be helpful.    Thanks.

https://technet.microsoft.com/en-us/library/bb124375%28v=exchg.150%29.aspx