Carbon Accounting: As I understand it this is the umbrella term for embodied carbon and operational carbon. It depends on what project you are building as to which is the most important consideration. For a pumping station you may which to consider lavishing you budget on some efficient pumps, as you operational carbon use will be high otherwise, in comparison spending an unnecessary amount on low carbon building materials will only save small amounts, so it is more efficient use of money, in terms of carbon (and in the case money) to put you budget in to efficient pumps. A store shed on the other hand will have low operational carbon, so green building materials may be a viable option here.
Emissions Factors: These are the basis of all carbon accounting; they tell you how damaging a process is in term of emissions per unit volume. It is important to use a standardized Emissions factors set, and a presume this is why, DEFRA have released a booklet (downloadable) with lots of emissions factors in it. However, so emissions factors are still on the drawing board, SWW get some of theirs from research groups.