City display question

[Upright]

Hi folks,

I've recently returned to Civ after buying the BTS expansion. Having fun so all's well

But I have one question regarding the city summary screen - where you can review the details of all your cities at once. There is one indicator that looks like a red coin, in between the Trade Route and GP points.

What is this indicating please?

 

[Lark]

Maintenance (rounded down).

 

[Upright]

Many thanks