Sideshift is the ability to move an attachment to the left or right (Nearside/Offside). What a sideshifter allow you to do is keep your existing attachment set up and add in the ability to sideshift (hook on variant). This is done by adding what is essentially a hydraulically controlled carriage onto your existing carriage and placing the attachment on the front. An integral sideshift on the other hand, replaces your existing none sideshift carriage, with a more flexible sideshift variant (The section lifted by the central mast lift ram).
It is important to note a hook on attachment will de-rate your truck more than an integral type, however you gain the flexibility to remove it when needed.