Difference between boundaries and rules

I was reading the Relationship Workbook and it talks about our expectations and rules we have on others, that will tell us a story/thoughts/feelings etc. often in a negative way when things aren’t met.

So how can I establish for myself what is a real boundary which is not being met, or whether it is just a rule that I can let go and meet my own need?

I read the list of what things people should do. I also go ahead and think: if I have to meet all my needs myself, what is then the point of being in a relationship almost?

Would love to have clarity and an insight on this to help me out and grow in this area, as I am working on relationships and unconditional love this month.