The ULX-D, in regular mode (10mW or 20mW), requires a guard band (on each side) of about 50% of the occupied channel bandwidth, which is about 175kHz. This means the adjacent channels' center frequencies must be ± 350kHz. 6MHz ÷ 350kHz ≈ 17 frequencies per TV channel.
When in high density mode (1mW and the PA is now extremely linear), the occupied channel bandwidth is 125kHz and no guard band is required. 6MHz ÷ 125kHz = 48 frequencies per TV channel.
When using IAS to coordinate in ULXD's, in the equipment definitions set the ULXD channel bandwidth to 349kHz (for regular mode) and the intermod spacings to 0. You'll need to do the same in Options when finding ULXD frequencies (remember to change the Options parameters back when finished).
I'm fairly certain the QLXD is the same RF section, just without the high density mode and user features.