block are vacant in the ip core of PCIe.

Since the Data and Descriptor are inputs to this block, its most likely a buffer / FIFO that stores both. Data & descriptors are for the SGDMA IP that the PCIe uses. When starting a PCIe transaction, the command and data have to be fetched from memory using the descriptor that is stored in the buffer. The software application controls the DMA, programs the descriptors and starts/stops the data transfer to the PCIe IP.