Plated-through slots, metal slots, and cut-outs refer to the same technology in PCB manufacturing. A plated slot means a slot is plated with copper. Actually, the plated-through slot is the plated-through hole pads with non-circular shape. Circular plated-thorough holes are the ideal choice for most through-hole components. But there are many components with rectangular or blade-shaped leads, which need the plated-through slots.
The process of plating the slot is the same with plating holes. First, to mill the slots, then do copper sink, and surface finish is the last step. And the slots have two kinds: plated-through hole (PTH) and non-plated through hole (NPTH).