A boundary definition survey is to define where the property boundaries are. These surveys are requested by the property owner for a number of reasons. It is often to accurately locate the boundary before carrying out any building additions or before construction of a new house or building. In some cases it is to locate the boundary before putting up a fence to ensure the fence goes in the correct location.
Sometimes there are conditions on a sale and purchase agreement that the survey pegs must be in the ground prior to settlement.