Red reticle range
From Halopedia, the Halo wiki
In the context of the Halo games, red reticle range refers to the distance at which a player's weapon reticle will turn red, when aimed at an enemy. This distance varies depending upon the weapon the player is currently wielding, and whether the player is hip-firing or zoomed in. The reticle being red indicates that when the player's weapon is fired, auto-aim will take effect, bending the fired projectile's trajectory towards the enemy.
The red reticle range is a useful metric for describing a weapon's effective range. If an enemy is beyond a weapon's red reticle range, the player will not receive any aim assist when firing the weapon, and so it is significantly harder to accurately hit them. The player can usually still deal damage at such distances, but reliably targeting the enemy is difficult, severely reducing the weapon's effectiveness.