They're a road map for how your relationship will work and how you will get your needs met.
When I worked with couples as both a Domestic Violence Victim Advocate and a Planned Parenthood Certified Responsible Sexuality Educator, I saw the problem that a lack of boundaries can cause. I even struggle with them in my own life, and I've had years of training on the subject.
Or maybe it's about if you'll go to a movie you hate in the spirit of compromise.
No matter how close you are, you'll both eventually need your space.
Not only will you need alone time, but you'll need solo time with your friends and families.
Or maybe the idea of your partner (or anyone) knowing your passwords makes you uncomfortable.
What you share is an important boundary, because if you don't set it, you could end up feeling violated.
However you like to communicate is fine, but there are some do's and don't's.
For example, if your partner insists you check in, and constantly calls or texts you when you're not together, it could be an issue of power and control, which is a red flag of an unhealthy relationship.
Talk with your partner about your expectations for alone time and solo time, and trust that it's healthy to be apart sometimes. You might be happy to post all the details of your romance online, but your partner might not.
And it could just be a matter of having co-workers and family members on social media that your partner doesn't want involved in your private lives. Or maybe you don't like the idea of your partner chatting with exes online.
This boundary comes down to respect, and it's all about personal preference.