Sc → Zn · Y → Cd · Hf → Hg · Rf → Cn
Many transition metals can adopt several oxidation states. That helps explain their colourful compounds, catalytic activity and rich coordination chemistry. Iron, cobalt and nickel are also closely associated with magnetism.
They are everywhere in technology: iron in structural steels, copper in electrical wiring, chromium in stainless steel, platinum-group metals in catalysts, and gold or silver in contacts and electronics.
The exact boundary of the category is not shown identically in every textbook. The central d-block is useful as a visual family, while stricter chemical definitions keep some edge cases separate.