Difference between fdt_addr and fdt_addr_r in uboot?

I have been working on implementing u-boot on a 64bit RPi4 for a few weeks.My last big mistake was that when booting the kernel in boot.cmd -> I was using the variable fdt_addr_r instead of fdt_addr.

Does anyone know the difference between the fdt_addr and fdt_addr_r variables?and why when loading the dtb we use fdt_addr_r ? and why we use fdt_addr when booting the kernel?

Thanks