The first operand specifies the location in memory to store the second operand. It can be any number from 0x0000 to 0xffff. However, the MCU may not have the entire range of memory available.
The second oeprand specifies which register to store. You can use any one of the 32 register.
The following instruction stores the value of register 23 to location 0x007e in memory.
sts 0x007e,r23