An agreement is a shared agreement that is non-violable. Unlike practices, they are not aspirational. They present a firm and clear boundary on how we choose to be with each other.
It's best to strive to keep agreements to a minimum for a few reasons: agreements are the most “expensive” agreements to change, often requiring the most work to revise or scrap. They’re also pretty reactionary and limit our ability to creatively problem solve and respond to new situations, eventually creating more challenges than they solve. Also, we live in a litigious culture that attempt to 'legislate co-operation'. Rule-creep can lead to a culture that externalizes common sense to a set of complicated policies instead of relying on relationships, and a shared vision to work/learn together.