Which of the following describes U-shaped patterns in fire investigation?

In fire investigation, U-shaped patterns are indicative of the behavior of fire as it spreads and interacts with the environment, particularly the presence of combustible materials, airflow, and structural features. A gently shaped line of demarcation suggests that there is a gradual transition between areas of differing fire effects, which aligns with how U-shaped patterns are formed.

These patterns typically occur when fire burns more intensely in specific areas due to factors like airflow or the presence of flammable materials, leading to a rounded, U-shaped appearance in the charred areas. This type of pattern can provide investigators with valuable insights into how the fire originated and spread, as they can indicate points of increased heat and potential sources of fuel.

The other options represent different types of burn patterns that do not align with the characteristics of U-shaped patterns. They offer distinctions in how fire interacts with various surfaces, but only the description of a gently shaped line of demarcation accurately captures the essence of U-shaped patterns in fire investigation.