Transformer: Double primary winding?
Some countries use 110VAC mains, others 220VAC. So, transformers would need to be constructed differently depending on the country from where it is used.

To avoid that, some manufacturers are selling transformers that have two primary windings. In 220V coutries, you wire the windings in series (as if the number of turns would double, thus doubling the voltage ratio), and in 110V countries, you wire them in parallel (as if the number of turns was the same, but with double wire section, thus double the current carrying capability).

With a single transformer design, they can sell everywhere.

Profit!

Moreover, with some switch arrangement, you can make the transformer primaries configuration selectable, so the same end product can be sold everywhere in the world (provided that the switch is configured correctly depending on where the product is distributed).

Profit again!