Slot populated means there is a card in that slot already. It looks like there is some sort of a mini-pci-express card in that slot. x1 is mini pci express.
Supermicro Motherboard MBD-X9SCL-B Intel LGA1155 DDR3 PCI-Express SATA 3.0G..., New Intel Centrino Wireless-N + WiMAX 6150BGN PCIe Half Size Mini Card 612BNXHMW, New Broadcom BCM2070 + BCM4313 802.11g/n WLAN + Bluetooth 3.0 PCI-E Wificard, Areca ARC-1210 Raid Controller pcie Card, Asus P9X79 DELUXE-Asus MB P9X79 DELUXE Core i7 LGA2011 X79 DDR3 PCIE3.0 SATA USB, Emulex OCE10102-FM -2CH 10GB PCIE OPTICAL UCNA - Kit, Atheros AR5B93 BGN Half WiFi MINI PCI-E For DELL M2400, Atheros AR9280 AR5BXB92 MINI PCI-E Wifi Card for HP DV7, eVGA 02G-P3-1559-KR Video Card GTX550 Ti DDR5 2GB PCI Express mini-HDMI-D-DVI, Gigabyte GV-N450-1GI-Gigabyte Video Card GV-N450-1GI GTS450 1GB DDR5 PCI-E VGA/D, 100281VX-2SR Video Card HD 5870 1GB VaporX DDR5 PCI-E Dual DVI-I-HDMI-DP Full, Asus M5A97-Asus MB M5A97 AMD AM3+ 970 SB950 DDR3 PCIE SATA 6GB s USB3.0 ATX Reta, Asus M5A97 EVO-Asus MB M5A97 EVO AMD AM3+ 970 SB950 DDR3 PCIE SATA 6GB s USB3.0 , Evga 01G-P3-1463-KR-VDO 1024MB PCI-E GeForce GTX 560 Superclocked, 256bit, GDDR5, Asus EAH6670/DIS/1GD5-Asus Video Card HD 6670 1GB GDDR5 128Bit PCIE DVI HDMI Dis, Supermicro MBD-X9SCL-B-Supermicro MB MBD-X9SCL-B Intel Socket 1155 DDR3 PCIE SAT, Asus P8B WS-Asus MB P8B WS Core i7 i5 i3 LGA1155 C206 NF200 DDR3 USB3 PCIE SATA , Emulex OCE10102-FX -2CH 10GB PCIE COPPER UCNA - Kit, NEW PCIE PCI-E PCIE SATA III 6G 2PORT + PATA RAID CARD WORK WINDOWS 7 32/64BIT, Intel E1G44ET2-Intel NC E1G44ET2 RET QUAD PORT PCI-E Copper 1Gb Server NIC Retai, Evga 015-P3-1582-TR -GEFORCE GTX 580 SC PCIE 2.0 - Kit, Evga 01G-P3-1561-KR -GEFORCE GTX 560 TI FPB PCIE 2.0 - Kit, Evga 01G-P3-1450-TR-eVGA Video Card 01G-P3-1450-TR NVIDIA GTS450 DDR5 1GB PCI-E , New SHARK® 1200W ATX12V EPS12 PC Power Supply PSU SATA/PCIe 20/24pin 140mm Fan, Visiontek 900327-RADEON HD5450 PCIE 512MB DMS59 - Kit, Visiontek 900315-Visiontek 900315 Radeon HD 5450 Graphics Card - PCI Express 2.1, HP WL048AT-HP WL048AT FirePro V3800 Graphics Card - PCI Express 2.0 x16 - 512 MB, PNY VCQ2000-PB -Quadro 2000 PCIe x16 1GB GDDR5 - Kit, PNY VCQ4000-PB-PNY VCQ4000-PB Quadro 4000 Graphics Card - PCI Express 2.0 x16 - , PNY VCQ5000-PB-PNY VCQ5000-PB Quadro 5000 Graphics Card - PCI Express 2.0 x16 - , Startech.com ST1000SPEX4-PCIe Ethernet Card - Kit, LSI Logic LSI00188 -LSI Logic 9200-8e SAS Controller - PCI Express x8, ATI Technologies 100-505605-FIREPRO V5800 PCIE 1GB GDDR5 - Kit, Visiontek 900308 -RADEON HD4350 X1 PCIE 512MB - Kit, Startech.com PEX312PUSB-3PORT USB PCI EXPRESS 12V - Kit, Lenovo 41W1027 Wireless Lan Adapter Intel Pro Wireless 3945abg Mini-pci Express, StarTech PEXSAT32 PCIe SATA Controller Card, D-Link DWA- 556 Xtreme N wireless PCI express desktop adapter, US Robotics USR5638-56K PCI Express Faxmodem - Kit, US Robotics USR8402-2PORT USB 3.0 PCIE CARD - Kit, EVGA E-GeForce 9600 GT 512 MB PCI-E, Belkin 3-Port Firewire Pci Express Card (F5U504), Startech.com PEX1TO162 -PCIE1 TO PCIE16 LP MOTHERBOARD - Kit, Evga 512-P3-1310-LR-VDO 512MB PCI-E GeForce 210 DDR3 1 DVI, 1 HDMI and 1 vga con, D-Link DUB-1310 2-Port USB 3.0 PCI Express Card - Netwo, Creative Labs SoundBlaster Recon3D PCIe, Visiontek Radeon 4650 PCIE 1GB HDMI, 4-port PCI Express LP USB Adapter Card, ATI Technologies FirePro V5800 1GB PCIe Giftbox, HP NVIDIA NVS 300 (512MB) PCIE X16 VGA CARD.,