Coordinate system for custom positioning system

[ar-ramchandra]

If I use my own motion capture system to calculate the 3d position of the crazyflie, and send it over to the drone via the api, what would it expect in terms of position ? What units must X,y,z be (I think it's meters but just want to make sure)?
If the drone is on the floor will its position be 0,0,0 or some X,y,0 (the last value is 0 because it's altitude is 0, but X and y are not because it's somewhere within my definition of a coordinate system) based on my definition of a coordinate system?

Can X,y,z be floats ? If so how many points of precision is accepted ?

Any advice is deeply appreciated!
Thanks in advance !