PDCCH order is used for the non-contention based random access procedure. It is sent by eNodeb to UE.
As part of carrier aggregation, UE does random access towards the secondary cells and it is only done by using the PDCCH order.
If UE receives PDCCH order masked with UE's C-RNTI and for a specific serving cell, UE initiates random access procedure towards that serving cell.
DCI format 1A is used by eNodeB to convey PDCCH order to UE.