The isoline calculation can be refined by specifying to the engine that some areas or roads are to be avoided during processing. Restricting the calculation in such a way will typically result in a smaller isoline, but since isolines cannot contain internal holes, the results might be unintuitive.
The request below calculates both a 25 km and 30 km isoline in the
short mode, near Berlin, Germany, with an option specified to avoid environmental zones. In the 25 km isoline it is clear that this area is avoided, but this is not the case in the 30 km isoline. The reason for this is that the extra distance allows routes to wrap around the city inner ring, closing the isoline beyond it. Since holes are not allowed, the whole area gets engulfed by the isoline.
curl -X GET \ 'https://isoline.router.hereapi.com/v8/isolines?transportMode=car&origin=52.6044796,13.4715151&range[type]=distance&range[values]=25000,30000&routingMode=short&avoid[zoneCategories]=environmental'