If a sub-project and parent project have different project currencies, this property should be set...

According to the ServiceNow documentation1, this property controls whether the cost roll-ups are performed when a sub-project and parent project have different project currencies. If this property is set to true, then the system converts the sub-project currency to the parent project currency using the exchange rate table and performs the cost roll-ups. If this property is set to false, then the system does not perform the cost roll-ups for sub-projects with different currencies.

1: https://docs.servicenow.com/bundle/vancouver-it-business-management/page/product/project-management/reference/r_Properties.html