Codex: Space Marines — Warhammer 40,000 3rd Edition Wiki

To choose units you need to look in the relevant section of the army list and decide what character, unit or vehicle you want, how many models will be in the unit, and what upgrades you want (if any). Remember, you cannot field models equipped with weapons and wargear not shown on the model. Once this is done subtract its cost from your army's total points, and then go back and make another choice. Continue this until you have spent all your points.

Force Organisation Charts

The army lists are used in conjunction with the force organisation charts from a scenario. Each chart is split into five categories that correspond to the sections in the army list, and each category may have one or more boxes. Each box indicates that you may make one choice from that section of the army list, while a dark-toned box means that you must make a choice from that section.

Note that unless a model or vehicle forms part of a squad or a squadron, it counts as a single choice from those available to your army.

Standard Missions Force Organisation Chart

Compulsory	Optional
1 HQ	1 HQ
2 Troops	4 Troops
	3 Elites
	3 Fast Attack
	3 Heavy Support

Diagram: Force organisation chart tree — HQ (compulsory) branching to Elites (optional), and two Troops boxes (compulsory) branching to two further Troops boxes (optional), a Fast Attack box (optional), and a Heavy Support box (optional).

The Standard Missions force organisation chart is a good example of how to choose an army. To begin with you will need at least one HQ unit and two Troop units (dark shaded boxes indicate units that must be taken for the mission). This leaves the following for you to choose from to make up your army's total points value: up to 1 HQ unit, 0-3 additional Elite units, 0-4 additional Troop units, 0-3 additional Fast Attack units or 0-3 additional Heavy Support units.

Army List Entries

Each army list entry consists of the following:

Unit Name: The type of unit and any limitations (if any) on the maximum number of choices you can make for it (eg 0-1).
Profile: These are the characteristics of that unit type, including its points cost. Where the unit has different warriors, there may be more than one profile.
Number/Squad: The number of models allowed in the unit, or the number of models you may take for one choice from the force organisation chart. Often this is a variable amount, in which case it shows the minimum and maximum unit size.
Weapons: These are the unit's standard weapons.
Options: Lists the different weapon/equipment options for the unit and any additional points cost for taking them. It may also include the option to upgrade a squad member to a character. If a squad is allowed to have models with upgraded weaponry (ie heavy weapons, plasma guns, etc) then these may only be taken by ordinary squad members, not the Sergeant.
Special Rules: This is where you'll find any special rules that apply to the unit.

For the official Q&A covering this codex, see Codex: Space Marines Q&A.