That depends on the type of ACC. The "old" one can only be activated above 30km/h. The new ones (ACC stop and Go) work until stand still. On of the reasons is that the old brake systems were too uncomfortable and noisy at low speed (the two piston pump is inducing too much vibration leading to an oszillating deceleration and noises).
Markus I appreciate your answer. In Stop and Go situation, according to my knowledge, if ACC is active, then only the system will keep track of speed of vehicle (if it is zero for few seconds or not).
But, say if I start the vehicle from my parking, and I want to activate the ACC system, without achieving any speed, will the system respond ?
You can. The software or processor should sense and discriminate your situation - from a "speedy -slower-ZERO" situation (the normal driving scenario) and a "Park-Zero-Slow" situation (Park-then-Go).