PLDA can only impose documents with UserUnit set to 1.
If documents with other UserUnit values are to be imposed, the value can be changed in this dialog and then set back after imposition.
UserUnit specification is here:
adobe_supplement_iso32000.pdf, page 129, Appendix C Implementation Limits, number 177.
pdf_reference_1-7, page 993.
In short, UserUnit acts as a dimension multiplier. From one millimeter when UserUnit is 1 there are two millimeters when UserUnit is 2.