With Relationships, there is not always a huge difference between topics covered under The Self (or in level 1). This is often more a continuation of lessons learned earlier, applying what we know to help with interacting with others. As others process information and look at the world in ways very different to ourselves, learning empathy and understanding of other are skills we need to develop to help with this.
One difference between this Relationships section (or level 2 information) and the level 3 information is scope. Another difference is who the focus is on. Not all relationships are healthy, or can be made to be such. This does take equal effort from any party involved. Sometimes the most important thing about a relationship is knowing when to work on it and when to stay away from the person. The information covered here will be focused on both the ideal as well as the dangerous types.
Stages of relationships and (friendship, courtship, newleywed, heads of family, etc)
Emotions and communication (humans are not logical)
motivations and hidden motivations. Conflicts between people
If you love others, understand them deeply. Pay attention.
Manipulation vs guidance and cooperation