"Threshold is relative to Ceiling. Variable from Ceiling value to 12 dB below Ceiling value. Automatic make-up gain is applied as the Threshold is lowered." That's what's inside the manual...
Let me try tro find a better explanation:
Basically you lower the threshold, like you would do with a normal compressor,
just that this compressor has a ratio of infinitive:1, which effectively turns it into a limiter.
As you turn down the threshold an equal amount of make-up gain is applied
to bring your peaks back up to the output ceiling. (Otherwise the overall level would just get quieter,
just like when using a compressor without adding make up gain.)
This logic makes actually more sense than most limiters where the "threshold" is infact not a threshold
but rather an input-gain knob, that raises the level towords the "ceiling"/"output".