Dubai-based airline Emirates recently revised its policy for paid-for seats.

According to information available on the carrier's website, preferred, twin seats and extra legroom seats have now been included in the paid-for seats policy, effective August 20, 2017.
Here are the details
Advance seat reservation on regular seats:

> Complimentary on or after ticket purchase for Economy Class Flex, Economy Class Flex Plus, Business Class, and First Class fares.

> Complimentary when online check-in opens, from 48 hours before departure for Economy Class Special and Economy Class Saver fares.

> Subject to a charge for Economy Class Special and Economy Class Saver fares (all tickets in booking classes X, V, T, L, Q and N) until online check-in opens.
Advance seat reservation on preferred and twin seats for all Economy Class fares:

Complimentary when online check-in opens, from 48 hours before departure.

Subject to a charge for all Economy Class fares until online check-in opens.
Advance seat reservation on extra legroom seats for all Economy Class fares:

Complimentary upon check-in at airport subject to availability

Subject to a charge for all Economy Class fares until check-in at airport

Source: Khaleej Times