Hi @incognito
While SPI in general is just the name of a type of communication bus, with this device we are usually referring to a SPI flash chip. In this case this chip is just a 16MB large memory chip. By default it currently is empty but it can be programmed with an image of U-Boot that can act as a bootloader and extend the native capabilities of the RK3588 SoC. By itself the RK3588 can boot from SD card, eMMC and this SPI flash chip. When a bootable image is available on the SPI flash it will be started and the image then can load some more drivers that allows booting from other devices/interfaces (e.g. NVMe, USB, Ethernet, etc.) or provide other functionality. 16MB is not much but sufficient for a basic bootloader.