[U-Boot] setting the environment address

How does uboot decide where in NAND the configuration will be found? I'm getting the classic errors: *** Warning - bad CRC or NAND, using default environment

I am writing my environment to nand, with crc, but how do I know that uboot is looking for it in the right place?

Is that a compile time only setting? Ultimately I'd like to have 2 copies of the environment.