Which feature allows the Ultimaker and the Form 2 machines to add material during a print if it runs low?

The feature that enables the Ultimaker and Form 2 machines to add material during a print if it runs low is known as mid-print material addition. This capability ensures that the printing process can continue seamlessly without interruption, even if the printer's material supply is dwindling.

When a machine is equipped with mid-print material addition, it can sense when the filament or resin is running low and either pause the print to allow for a material change or automatically engage an additional material supply. This feature is particularly crucial in professional printing environments where maintaining print continuity is essential, minimizing wasted time and resources.

Other options, while noteworthy in their own right, do not specifically address the functionality of adding material during a print job. Automatic filament detection typically refers to the machine's capability to detect the presence or absence of material, but it does not imply active addition during printing. Structural support generation pertains to creating necessary supports for overhangs in a print, and color change capability allows for different colors to be introduced but does not pertain to handling material shortages. Therefore, mid-print material addition is the most fitting feature for this context.