Negative "City Use" Production

[Maquiladora]

When ive got my PW Tax at 100% sometimes the "City Use" figure goes into negatives, what does this mean? Is it taking away production already spent on items in cities?

[Fromafar]

Unit maintenance costs. And no, it gets deducted from the public works.

[Maquiladora]

Question asked, question answered. I guess its fairly obvious with it being under the same line as Public Works figure, but you dont get anywhere without asking questions