I imagine you mean the MFD while flying regarding "H" or "CTRL+H".
Target lock depends upon weapon system, range to target, and whether or not the weapon can actually "lock" onto the target. Which airframe and weapons are you asking about?
AH-6 Cannon and 70mm Rockets
AH-64 Cannon, Hellfire "L" model, Hellfire "K" model, AA Missiles