Do you mean onchip RAM and XRAM
The architecture of the '51 does not allow more than 256 bytes of what, unforunately, commonly is referred to as RAM, where, correctly, it should be referred to a "direct RAM" and "indirect RAM".

Now, sone very nice derivative designers have included more RAM on the chip which, although on chip, must be addressed as XRAM. There is, at this time no consensus as to the name of this "internally located external RAM" some use my preference ERAM and others use IRAM.

As long as the DATA address is less than the size of the ERAM a MOVX will access internal memory, beyond that it will access external memory.

Erik