The boundary line is where the property ends. An easement is aright to cross over your property. Most easements are for utilities or access to a property. Both are pretty much written in stone and beyond your ability to control.
The Boundary Line Agreement (BL AGR) provides a procedure for County review whenever a point or line determining the boundary between two or more parcels of real property cannot be identified from the existing public record, monuments, and landmarks or is in dispute. RCW 58.04.
Boundary lines help determine the extent of a property and its legal ownership. Specifically, these lines define a property's physical limits and help prevent conflicts between neighboring property owners.
Under such an agreement, the parties acknowledge the true boundary line between the properties, and the encroaching owner releases any claim to the strip of land encroached upon. In return, the “encroached upon” owner allows the encroachment to continue for so long as he/she is not adversely affected.
You and your neighbour can create a 'boundary agreement' to record: the boundary between 2 properties. who's responsible for maintaining a hedge, wall, tree or fence between 2 properties.