The clearance is a perfectly valid one and quite standard in the US. It simply means that you climb to 15000’ whilst complying with altitude constraints such as above or below altitudes at fixes. This may require you to climb quickly or to level off for a while until passing a certain waypoint. On the Orcka this might mean crossing at or above 13000 at KEGGS for example. It also means reach 15000 by COOPP