Reality quotient

A reality quotient, or RQ, referred to how "real" something was. Objects native to the space-time continuum had a reality quotient of 1, where they could fully participate in events around them and make a difference. Objects with an RQ of 0.7 were still real enough to divert the course of history. Patrons of the Crystal Bucephalus were projected in time with a reality quotient of 0.5, anything less making them phantoms or hosts. But at the "half real" level, 0.5 allowed the patrons to eat, drink, defecate, talk, kill, maim, marry, impregnate, etc., and then return to the Bucephalus with the time stream "none the wise," as the Fifth Doctor explained; "Too small a pebble to divert the river of destiny." (PROSE: The Crystal Bucephalus)

When the Master used his TARDIS to try and steal the TITAN Array, the Sixth Doctor used his own TARDIS to put a time lock on the Array, boosting its reality quotient above 1 and embedding it deeply into the space-time continuum. As the Master's TARDIS tried to pull away, the Doctor boosted the temporal fields, increasing the depth of the time lock to an RQ of 3. As the Quantum Archangel created parallel universes for all seven billion people on 2003 Earth, the Doctor used a reality inhibitor to boost the TARDIS's reality quotient to about 10, which he explained should make the formation of parallel universes a bit tricky, even for her. (PROSE: The Quantum Archangel)