This occurs when DxO PhotoLab deems a focal length to be ambiguous.
When manual adjustment of focal length is possible (using manual focus control), DxO PhotoLab analyzes the images and determines that several positions of the focus ring are indicated by the same focal length value in the EXIF. Due to this DxO PhotoLab cannot automatically determine the correct focal value and requires user input.
In most cases these focal values differ only slightly (for example on a long zoom lens), so we do not declare the focal length to be ambiguous, and no user input is required.
If those measures are sufficiently different, DxO PhotoLab requires manual input. The software then marks the focal value as ambiguous and prompts the user to fine tune the distortion corrections.