I do not understand this rule. Rule 38.1 has any figure upgrades in the front rank. 38.6 says that you can only assign damage to figure upgrades that are not in the back rank. Is the mention of the back rank redundant or is there some reason for the comment that I have overlooked?

38.1 is straightforward. Figure upgrades must be on trays in your front rank.

This protects them somewhat as damage must be applied to models in the bank rank meaning they can't be targeted until you only have your front rank left.

However, rule 38.6 "Attackers can assign damage to figures that are not in the backmost rank by spending one or more accuracy icons during attacks." provides an exception to the normal way damage is applied.

Say you have a big block of 3x3 Spearmen with an Artifact Bearer (Defence 3/Wounds 2) in the front rank which is attacked by a 3x2 unit of Reanimate Archers.

The Archers roll exceptionally getting hit/hit on the red die and hit/accuracy on the blue die; with their threat of 3 that's 9 damage.

Normally, all the damage would be assigned to models in the back rank but because the Archers rolled 1 accuracy rule 38.1 allows them to spend it in order to apply enough damage to cause 1 wound to ANY model in the unit regardless of what rank they are in.

So in this case they could spend 3 of the damage on the Citadel Weapons Master in the front rank to wound him once, however they can't kill him as because they only had 1 accuracy result to spend the remaining 6 damage must be applied to models in the back rank as normal.

Hope this explanation helps.

As far as redundancy, I kind of see your point. However, when there is only one rank remaining, it is considered both the front and back, so in those cases you would have figure upgrades in the backmost rank (66.1). By the sound of it, you cannot spend accuracy icons to target figure upgrades if the target unit is down to one rank. This is irrelevant at the moment (because you can assign damage to those figure upgrades any way you want if there is only one rank), but if future game effects trigger off of spending accuracy, this will matter.

38.6 isn't prohibiting you from spending accuracy tokens on units in the backmost rank. It is telling you that you are able to assign damage to figures in other ranks if you spend an accuracy token.

The section on Accuracy on page 6 of the RRG doesn't make any mention of ranks in describing how to use an accuracy token, allowing the accuracy token to be assigned to any figure upgrade. 38.6 is just calling out the use of accuracy tokens as being an exception to how damage is assigned.

RRG, pg. 11:
'38.6 Attackers can assign damage to figure upgrades that are not in the backmost rank by spending one or more accuracy icons during attacks.'

RRG, pg 6:
'2 Accuracy

Accuracy is a die icon that allows attackers to wound figure upgrades during an attack.

2.1 When resolving an attack, for each accuracy icon the attacker rolls, he chooses one of the defender’s figure upgrades and assigns that accuracy icon to that figure.
2.2 A figure upgrade can have multiple accuracy icons assigned to it.
2.3 If the defender has no figure upgrades, the attacker cannot assign any accuracy icons; they are ignored.
2.4 During the “Spend Mortal Strikes” and “Spend Hits” steps of an attack, the attacker can assign wounds and damage to figures that have been assigned an accuracy icon, even if they are in a tray that cannot normally be assigned wounds or damage.
2.5 Each time a wound is suffered by a figure that has an accuracy icon, one accuracy icon on that figure is spent.
2.6 When a figure upgrade is removed from a unit, any remaining accuracy icons assigned to it are ignored.'