This occurs when DxO PhotoLab 4 deems a focal length to be ambiguous.
When manual adjustment of focal length is possible (using manual focus control), DxO PhotoLab 4 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 4 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 4 requires manual input. The software then marks the focal value as ambiguous and prompts the user to fine tune the distortion corrections.