What primary observation marker is associated with 'Oumuamua?

Extreme elongation and non-gravitational acceleration

When comparing the primary observational markers of the three known interstellar visitors, 'Oumuamua, the first detected, is characterized by two distinct traits. The comparison table specifies its primary observation markers as exhibiting extreme elongation, referring to its highly unusual, elongated shape, and the detection of non-gravitational acceleration. This acceleration suggested forces other than just the Sun's gravity were influencing its trajectory, a phenomenon often discussed in relation to outgassing in comets, though 'Oumuamua itself was never definitively confirmed as cometary based on traditional tail appearance.