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Work has a strict definition. It is a the scalar result of scalar multiplication of a force vector times its displacement vector for an event.
Often, writing at elementary levels about "work" an author will want to address work not as energy transfer of a system, as independent system aspect. That is without the work being part of an energy equation of a system. To take such an approach the author must specify the "direction" of energy transfer. Hence work done on (work done by) a system means the system energy increased (decreased).
More to be written here!