A strategic petroleum reserve is crude held by a government for emergency use rather than for commercial purposes. The US SPR, stored in salt caverns on the Gulf Coast, is the largest, and IEA member countries collectively hold obligations to maintain emergency stocks.
The first thing to be clear about is that strategic stocks are not commercial inventory and should not be added to it uncritically. Commercial stocks are held by companies responding to price and carry, so they respond to curve structure and drain in backwardation. Strategic stocks are held by a state on a policy rule, and they respond to nothing except a political decision. Summing the two produces a total that behaves like neither.
A release is a sale or a loan of crude into the market. Mechanically it adds supply for a period, and the market reaction depends heavily on the size relative to the disruption it is answering and on whether it is understood as one-off or sustained. Because the volume is finite and known, a release is more usefully thought of as buying time than as changing the balance: it shifts barrels forward in time and creates a future refill obligation.
Salt cavern storage has a specific physical characteristic worth knowing: crude is displaced with water, and the caverns degrade with each fill and draw cycle. Capacity is not perfectly reusable, and drawdown rates are limited by the physical infrastructure rather than only by policy. A reserve's nameplate volume overstates what can be delivered in a given week.
China's reserve programme is a distinct case and is covered separately, because the volumes are not published and the buying behaviour is inferred rather than observed. See China's strategic reserves.