What is $SYS_TEXT_BASE and where to find it for a board?

Rockchip Boot Option document says:

When using idbLoader from Rockchip miniloader, need package u-boot.bin into miniloader loadable format by Rockchip tool loaderimage.

tools/loaderimage --pack --uboot u-boot.bin uboot.img $SYS_TEXT_BASE

Where SoCs may have different $SYS_TEXT_BASE.

What is SYS_TEXT_BASE and where does it come from?

From rockchip-bsp/build/mk-uboot.sh I see that for RK3308 SYS_TEXT_BASE = 0x600000, but why? If I don’t have this predefined mk-uboot.sh script, then how can I find the proper value for a particular board (for example, for Rock Pi S)?