What is PCIe to BAR Requests mean?

Hello Guys, I was using Nsight Graphics GPU trace mode to capture GPU performance metrics. In my case, I noticed that there are heavy PCIe throughput during rendering and there are a lot of PCIe to BAR Requests. FYI, My app uses a lot of VRAM (usually # GB over dedicated VRAM budget). So I was guessing that the PCIe throughput was caused by dedicated VRAM and shared system RAM swapping data. Can someone explain this? Many thanks.