Force Field, the spell. Force Fields, the rule some Retribution warjacks have.

Rules Clarification : Force Fields      (Edit) You cannot spend focus to heal Force Fields mid-move, such as taking damage from a free strike. (Infernal Ruling) You cannot repair force fields, but you can spend focus to remove damage. (Infernal Ruling) Grievous Wounds prevents removing damage from force fields. (Locked Thread)

Official Rules

Page 58 of the 2021.08 version of the rulebook pdf

Some models, notably the Retribution warjacks produced by House Shyeel, have two damage tracks: a set of boxes representing their force fields and another representing their damage grids. Mark the field boxes before marking the damage grids.

EXAMPLE: When damage is dealt to column 2, mark damage first in the force field boxes and then in column 2 if the force field is filled (even if the column was chosen by the attacker as a result of a special rule such as a Widowmaker Scout’s Marksman rule).

Fields & Simultaneous Damage

When damage is dealt simultaneously to each column, mark damage to the force field boxes first (damage from the first column, damage from the second column, and so on). If all of the field’s boxes are filled, mark remaining damage to the last columns.

EXAMPLE: Gorten Grundback hits a warjack with a force field with his Molten Metal spell. That spell deals 1 point of damage to each column on the warjack’s damage grid; the damage grid has six columns, so the spell deals 6 damage points. If the warjack has three unmarked force field boxes remaining, the damage points that would otherwise be marked in the first three columns are marked in the force field instead. The remaining 3 damage points are assigned to columns 4, 5, and 6.

Damage to a specific system is marked in that system; in this case, the force field is ignored.

EXAMPLE: If a warjack with a force field is hit by a Lancer’s Shock Shield attack, the warjack hit suffers 1 damage point to its first available Cortex system box. This damage is applied to an unmarked Cortex system box, not to the warjack’s force field.

Removing Field Damage

Once per turn during its activation, a model with a force field can spend 1 focus point to remove 5 damage points from its force field damage track unless its Field Generator (G) system is crippled.