Which type of appliances is referred to as portable hydrants and used when larger volumes of water are needed?

The type of appliance referred to as portable hydrants that are used when larger volumes of water are needed is typically a water thief. This device allows firefighters to draw water from a single source and redirect it into multiple lines, facilitating the efficient use of larger volumes of water in firefighting scenarios.

Comparatively, larger diameter hose appliances serve different functions, primarily focused on facilitating the transport and delivery of water through larger hoses rather than acting as a point for multiple connections to a water source. Other options like Siamese appliances and gated wyes each serve unique purposes related to hose management and water flow control but do not fit the criteria of being portable hydrants designed specifically for drawing large volumes of water for multiple lines. The functionality of a water thief aligns perfectly with the need for enhancing fire suppression efforts using substantial water supply.