As Tomb Kings armies can make wide use of chariot units, here is a summary and some clarifications of the chariot unit rules:
Chariots rank up just like infantry and cavalry models, and benefit fully from any command models they may have.
A character model inside a unit of chariots will only benefit from the "Look Out, Sir!" rule if the unit includes five or more rank and file chariots.
Wounds on a Chariot unit Champion do not carry over into the rank and file, and vice versa. In other words, the rank and file lose a chariot for every 3 wounds suffered, and the unit Champion is destroyed once he has suffered 3 wounds.
Chariots can only be joined by characters also on chariots, and characters on chariots may only join chariot units. This means that if the character is with a unit when his chariot is destroyed, from the end of the phase he is no longer part of the unit (you might like to place him 1" away to make sure this is clear).
If a chariot is wounded in close combat by a weapon that has Strength 7 or higher it is automatically destroyed. In this case, all the wounds that the chariot had remaining are counted towards combat resolution.