This parameter sets all the cities to display in the search panel.  Normally, this would be set globally in Rover IDX >> Styling >> Search Panel >> Cities.  This parameter is really only necessary if you want to override the global setting for a specific page.

It's important to understand the difference between all_cities="" and cities="".

all_cities= defines the cities that appear in the Cities dropdown in the search panel.

cities= defines the currently selected cities