FoC removed, percentages added.

Players have a "sideboard" of up to X number of selections (2-5, bracketed on points, so 1k or less games you have 2 sideboards, at 3k+ you have 5).

Sideboards can't be more than 25% of the total, or can be none at all.

They are referred to as "Secondary Detachments."

They are used for anything from allies to just additional things from your own codex.

If they are allies, then they require an HQ and a troop, and are still bound by the 25% of total.

Both players are expected to have sideboards.

Sideboarding is now a part of the game, done before deployment but in order of turn priority.  So the person going first, picks their sideboard first after learning what race their opponent is playing, and seeing 75% of their army (and the available sideboards).

The person going second then picks their sideboard, after their opponent has selected, but before either side deploys.
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Other tweeks include assaulting as a form of sweeping advance/consolidation.

The option to flee, in response to being charged (after overwatching) but there is the potential to be swept and the unit charging can (if they have the movement and sweep you) just hit a different unit provided it's in the same rough direction as the unit they swept.

The main thing I wanted to touch on is sideboards, % based army building, battle brothers being removed and units being able to lock themselves into combat to combat, but simultaneously enemy generals having another tool to avoid combat to counter act this.

Just to be clear so you have no misconceptions about what sideboarding is, (and I will have more details here in a bit as I get them sorted.) Sideboarding is building 2 or more alternative add on lists to your main force. There can be no more than 25% of your list and are chosen after the opponent reveals their list. Its now part of gameplay.