1)(2
		 3)(4
		 5)(6
		 7)(8
		 9)(10
		11)(12
DAT3	13)(14
CLK		15)(16	CMD
		17)(18	DAT0
		19)(20
		21)(22	DAT1
		23)(24
		25)(26
		27)(28
		29)(30
		31)(32
		33)(34
		35)(36
DAT2	37)(38
		39)(40

SDIO is the SD host/eMMC interface on the Raspberry Pi. SD host signals are normally used for the microSD slot.
These pins are "SD host" on Alt0 and "eMMC" on Alt3

have yet to actually get this to work, there are conflicting pinouts on:
https://ralimtek.com/raspberry%20pi/electronics/software/raspberry_pi_secondary_sd_card/