I want to say better to be thought of ... but that would be...

I want to say better to be thought of ... but that would be unkind. This is a sight to enrich the mind and the wallet.

NPV simply reflects the cost of money, the opportunity cost of money, and that money recieved in the future is worth less to someone than money received today  (bird in the hand and all that). The discount rate with which an NPV is calculated determines how much less money received in the future (if it's received at all) is worth to someone today. At the very least money in the future needs to be discounted by what return money today can earn in a risk free government bond while as an alternative to waiting for that money in the future. Say a discount rate of 3% per anum. Of course the bond rate already takes account of the expected inflation rate.

However, money promised by mining venture in Africa is obviously more risky than a government bond. Therefore the profits expected in the future need to be discounted by much more than the 3% of a guvvie, more like 12% reflecting the greater risk and uncertainty that the money wil actually appear as promised. The higher the discount rate the less future money is worth to calculated back to today. A higher discount rate is one way to reflect the greater risk and uncertainty that future money will be paid compared to getting that money in the hand today. 

Therefore a Scoping Study future mine profit will attract higher discount rate (say 15%)  than a Pre-feasibility study rate of say 12%, compared to a Definitive Feasibility discount rate of 8%. The lower discount rate makes future returns today worth more, reflecting the lower risk that they will appear as per the study projection. Gold projects have traditionally used 5% for DFS studies because there has been the assumption that gold is a hedge against market/economic risk and therefore has inherently built in upside should world events turn pear shaped. Not reasonable in my view but tradition is hard to break with when it suits those using it obviously.

Resources and Reserves

The terms used in resources and reserves are different to ensure they are not mixed up or confused.

Resources are either: Measured, Indicated or Inferred

Reserves are either: Probable or Proven (possible was removed long time ago)

The categories reflect the level of confidence as the wording indicates (no pun intended) and are NOT mutually inclusive. That is to say, the amount of Measured Resources does not overlap with Indicated or Inferred resources, they are added together for Total Resources.

Probable or Proven Reserves are what a mine study says can be profitably recovered from a deposit, that amount of ore IS mutually inclusive of the Resources. Reserves reflect the proportion of resources converted into mineable reserves. The Reserves will by definition always be less than the Resources. A good deposit has a high conversion of resources to reserves, a poor one not so much. 

Mates here, off to the pub. Hope this helps.