Glojo,
In CAS, 'Close' refers to the close integration of air platforms with ground forces rather than a proximity issue. The integration and situational understanding is what prevents blue on blue most of the time and it is a two-way responsibility. The perfect example is a chap in Creech AFB, Nevada, consenting release of a weapon from a UAV in Helmand having built a close rapport with a controller on the ground there.
Regards,
ICBM