Most phrases can be understood by thinking about what each word means.
General meaning
Back means rear, or farthest away
Boundary means a specified line separating two places.
A line is often thought of as straight, but can be an uneven line. A line also delineates areas.
So a "back boundary line" is a specified boundary at the rear.
Your back yard has a "back property line" as does a tennis court.
Note: Also see Tennis terms.
the back boundary line, and the long service line
The back of a badminton court is a back boundary line and long service line for singles.
The boundary line is solid. If not it will be a dashed line.
In volleyball, you serve from behind the back boundary line on your side of the court.
The boundary line marks the edge or limit of a defined area. The area immediately inside the boundary refers to the space or region that falls within the set limits of the boundary line.
The official definition of boundary is "a line that marks the limits of an area; a dividing line."
The boundary line under the basket is known as the baseline.
If a volleyball hits the boundary line, it is considered in bounds.
A boundary is a restriction, a line of demarkation. That fence is the boundary between our property. The ball went outside of the boundary.
the ball went outside of the boundary line.
the treaty line
The boundary of the set of rational numbers as a subset of the real line is the real line.