No one uses the old PCI slots for video. I tried it once, about eight or nine years ago, with a very old business-type PC that had AGP slot for video cards. I bought a used PCI video card and installed it, and it sucked big time. Much worse than current integrated video. I never used it because even nine years ago it couldn't play any games.
If you're asking about a selection in BIOS, then it is referring to a discrete video card (PCIEx) or something else, possibly PCI. If you have a discrete video card and your computer is no older than seven years old, choose PCIEx, that's what you are using.