How does this doctrine work can some one explain it to me?

Everyone (well the PC's at least) gain +02 bonus to WS characteristics. So once characters are created normally, add this in too.

All PCs in the unit also gain the Talent: Street Fighting (see p. 106 for description). Add 1/2 weapons skill bonus to crits when using knives, small weapons and the like or hand-to-hand.

Also for regimental equipment loadout, a minor weapons upgrade by adding mono-edge to weapon or trade-out with a common availability item. Mono edge grants +02 to penetration (p. 135).

Hope this helps.

so common in this context is craftsmanship or availability of item in either case it seems very open what is the intent behind the wording of the equipment being replaced.

They mean availability. Which could mean that you have a good quality plentiful item.

Depending on other choices, the "standard melee weapon" that is to be replaced might well be the knife, yes?

And it can be replaced by ANY regular weapon of Common availability?

Like the M36 Lasgun?

And it can be replaced by ANY regular weapon of Common availability?

Any Primary Weapon, which I take to be a typo/artifact - Black Crusade called the former Primitive melee weapons Primary. Only War calls them low-tech, which is what this section probably should be changed to (IIRC, I already noted this in the proofreading section).

Cifer said:

They have fixed this in the first beta update. You can replace the regiments standard melee weapon [knife] with any Low-Tech melee weapon of Common Availability.