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It is a process to remove the unused section of plated through holes.
This will ensure signal stubs are minimized; Stubs are the source of impedance discontinuities and signal reflections which become more critical as data rates increase. It is preferred method in HS designs.
Another reason for backdrilling instead of using blind and buried is if you are using it in a backplane, you most likely will not have many vias as you will be going from connector to connector so you would be backdrilling the connector pins which wouldn't be blind or buried.
in the designs i was talking about we use press-fit connectors so no soldering required. The files we typically send are just modified drill files that give the coordinates of the holes to be back drilled they also include the a fab drawing with the depth of each drill required.