The presence of a natural resource anywhere on Earth is known as an occurrence. This means it includes natural resources that are known to exist or not yet found (and may never be found). Therefore, the total occurrence of a resource is "how much" of a certain resource exists on Earth. This is an important idea since a fossil fuel's total occurrence will deplete every time it is extracted. When energy companies discuss natural resources, the focus is on the reserves rather than the total occurrence, because there's no interest in an occurrence that isn't profitable. The quantity in the reserve is known, but always fluctuates. The resource's total occurrence is limited (for most resources, see next section for more info). The total occurrence of fossil fuels or other mineral deposits don't increase because the usage of natural resources is so much faster than the time it takes for them to form (see fossil fuel formation).
An occurrence can refer to a fuel or flow (See fuel vs flow for more information). Although a fuel's occurrence is difficult to truly quantify, a flow's occurrence is often known. For example, the total occurrence of wind is all of the wind that's blowing on the planet at any moment. Renewable energy technologies (ex. solar power and wind power) can only be used when these resources occur. The occurrence of fuels and flows play an integral role in powering a high energy society through energy conversion technologies. A flows occurrence is largely independent of whether or not its used (because it just "exists"), a fuel's total occurrence decreases every time it is extracted. Although quite extensive, the fuels that power societies have a total occurrence that is limited.