ADC128D818 readout data format

 The ADC128D818 is a 12 bit A/D convertor.
The datasheet insists that output of the device is mapped in 16 bit register (two 8 bits registers).
Can you let me know how 12bits data are mapped on to 16 bit register
Is the 12 bits data is mapped MSB stuffing or LSB bit stuffing, like “0000b+12 bit data” or
“12 bit data + 0000b”?

When 2.5V input is applied to the device, the readout data was 0xF9A0”.
The data seemed to be LSB stuffing.