What is a key characteristic of a portable tank?

A portable tank is designed specifically for ease of transport and versatility in use. This characteristic allows firefighters to deploy water storage quickly at locations where a fixed supply may not be available. Unlike a stationary tank that is usually fixed in place, a portable tank can be set up rapidly at the scene of an incident and moved as needed, making it particularly useful in firefighting operations where access to water is critical.

In relation to the other options, a portable tank does not inherently require a static water source; it functions as a temporary reservoir for water and can be filled from various sources. Additionally, while some portable tanks can be carried or moved by firefighters, they are typically not designed to be "carried" on an apparatus like a standard water tank would be. Lastly, there is no requirement for a portable tank to remain filled at all times; it can be filled and emptied based on the needs of the situation, further emphasizing its flexibility in firefighting operations.