The desk and wall mode adjust for boundary gain. Desk mode for the desk surface in front of the speaker; wall mode for the wall behind the speaker. Boundaries act as acoustic mirrors. The direct signal from the driver interferes with the reflected signal from the boundary. Those settings provide a degree of compensation.
You've read the table below? Click on it to read.
House of the Rising Sone Out in the mid or far field Dedicated mid-woofers are over-rated