Sergeant Telion

Sergeant Telion may only join units of Scouts

Shots fired by Sergeant Telion (excluding Snap Shots) are always Precision Shots

If Sergeant Telion does not make a shooting attack or Run in the Shooting phase, you can nominate one model in his squad.  That model can use Sergeant Telion's Ballistic Skill that phase.

A unit that contains at least one model with this special rule automatically passes Fear and Regroup tests. When it Regroups, the unit does not make the 3" Regroup move, but can instead move, shoot (or Run) and declare charges normally in that turn. Furthermore, if a unit containing one or more models with this special rule is caught by a Sweeping Advance, they are not destroyed, but remain locked in combat instead.

When choosing an army, all models in the same Detachment or Formation must be drawn from the same Chapter.  If a unit's datasheet contains a Chapter in brackets after the Chapter Tactics special rule - for example, "Chapter Tactics (Ultramarines)" - the unit must always be drawn from this Chapter, and therefore cannot be included in a Detachment or Formation with any units that are drawn from other Chapters. All models drawn from a given Chapter benefit from that Chapter's Special Rule. If a unit contains models drawn from two different Chapters, it counts as from neither Chapter, and thus benefits from neither Chapter Tactic.

Independent characters can join other units.  They cannot, however, join units that contain vehicles or Monstrous Creatures.  They can join other Independent Characters, though, to form a powerful multi-character unit!  Independent Characters pass Look Out, Sir rolls on a 2+. A unit that contains one or more Independent Characters does not need a double 1 to Regroup if reduced to below 25% of its starting numbres, but instead tests as if it had at least 25% remaining.

Full details in BRB. Rule too long to copy verbatim.

Units that contain at least one model with this special rule are deployed last, after all other units (friend and foe) have been deployed. If both sides have Infiltrators, the players roll-off and the winner decides who goes first, then alternate deploying these units.

Infiltrators can be set up anywhere on the table that is more than 12" from any enemy unit, as long as no deployed enemy unit can draw line of sight to them. This includes in a building, as long as the building is more than 12" from any enemy unit. Alternatively, they can be set up anywhere on the table more than 18" from any enemy unit, even in plain sight.

If a unit with Infiltrate deploys inside a Dedicated Transport, they may Infiltrate along with their Transport. 

A unit that deploys using these rules cannot charge in their first turn. Having Infiltrate also confers the Outflank special rule to units of Infiltrators that are kept as Reserves.

If a unit has both the Infiltrate and Scout special rule, that unit can deploy as per the Infiltrate special rule and then redeploy as per the Scout special rule.

A unit that contains at least one model with this special rule rolls an extra D6 when rolling to move through difficult terrain and is not slowed by charging through difficult terrain. In most circumstances, this will mean that, when moving, the unit rolls 3D6 and picks the highest roll. Furthermore, a model with the Move Through Cover special rule automatically passes Dangerous Terrain tests.

After both sides have deployed (including Infiltrators), but before the first player begins his first turn, a unit containing at least one model with this special rule can choose to redeploy. If the unit is Infantry, Artillery, a Walker or a Monstrous Creature, each model can redeploy anywhere entirely within 6" of its current position. If it is any other unit type, each model can instead
redeploy anywhere entirely within 12" of its current position. During this redeployment, Scouts can move outside the owning player's deployment zone, but must remain more than 12" away from any enemy unit. A unit that makes a Scout redeployment cannot charge in the first game turn. A unit cannot embark or disembark as part of a Scout redeployment. 

If both sides have Scouts, roll-off; the winner decides who redeploys first. Then alternate redeploying Scout units.

If a unit with this special rule is deployed inside a Dedicated Transport, it confers the Scout special rule to the Transport (though a disembarkation cannot be performed as part of the redeployment). Note that a Transport with this special rule does not lose it if a unit without this special rule is embarked upon it. Having Scout also confers the Outflank special rule to units of Scouts that are kept as Reserves.

If a unit has both the Infiltrate and Scout special rule, that unit can deploy as per the Infiltrate special rule and then redeploy as per the Scout special rule.

A unit that contains at least one model with this special rule counts its cover saves as being 1 point better than normal. Note that this means that a model with the Stealth special rule always has a cover save of at least 6+, even if it is in the open. This rule is often presented as Stealth (X) where X indicates a specific type of terrain, such as Stealth (Woods) or Stealth (Ruins). If this is the case, the unit only gains the benefit whilst it is in terrain of the specified type. Cover save bonuses from the Shrouded and Stealth special rules are cumulative (to amaximum of a 2+ cover save).

If a model with this special rule rolls a 6 To Hit with a shooting weapon, that shot is a 'Precision Shot'.

Wounds from Precision Shots are allocated against a model (or models) of your choice in the target unit, as long as it is in range and line of sight of the firer, rather than following the normal rules for Wound allocation. A character that has a Precision Shot Wound allocated to it can still make a Look Out, Sir roll.

Note that Snap Shots and shots from weapons that scatter, or do not roll To Hit, can never be Precision Shots.