According to SKYbrary:
A conditional clearance is a clearance issued by an air traffic controller which does not become effective until a specified condition has been satisfied.
Example of conditions that may be included in a conditional clearance include:
- AFTER THE DEPARTING AIRCRAFT;
- BEHIND THE LANDING AIRCRAFT;
- BEHIND THE LUFTHANSA A340.
Conditional clearances are allowed and used by some countries, provided the aircraft or vehicles concerned are seen by the appropriate controller and pilot, but they are not allowed in the United States. Which is why they are not used in Infinite Flight because we follow the FAA’s rule book, currently.
3.7.1 — GROUND TRAFFIC MOVEMENT
a) Do not issue conditional instructions that are dependent upon the movement of an arrival aircraft on or approaching the runway or a departure aircraft established on a takeoff roll. Do not say, “Line up and wait behind landing traffic,” or “Taxi/proceed across Runway Three-Six behind departing/landing Citation.” The above requirements do not preclude issuing instructions to follow an aircraft observed to be operating on the movement area in accordance with an ATC clearance/instruction and in such a manner that the instructions to follow are not ambiguous.
b) Do not issue unconditional instructions when authorizing movement on a runway/taxiway for the purpose of airfield checks or other airport operations. Instructions must ensure positive control with specific instructions to proceed on a runway or movement area, and as necessary, hold short instructions.