I'm pretty sure the Alignment has to do with how empires will react after the diplomatic discussion is over. Hence Buerocratic Imbligo, Cold War, and War have bad alignments since they hurt the other faction and will cause the other faction to react badly (negatively). While Peace, Trade & Science Agreements and Map sharing have Good Alignments since they cause the other faction to react positively/favorably to those diplomatic actions/treaties. 

It really is just there to help you make decisions and tell you how those discussions will affect diplomatic relations with another faction

For pressure I don't think it does, b/c both Trade/Science Agreements can increase your pressure on another faction, while War can also increase your pressure (that is if you're winning it)