It a common practice that the starting point

It can be seen that international investment is a highly regulated are, however still there is no clear conclusion in terms of the application of treaties. There are many dispute has been settled to clarify  certain issues with regards to the rights on investors, but some of them are still differ. 
To interpret a treaty, there is a common practice that the starting point is the Vienna Convention as the ordinary meaning shall be regarded first. To define investment and investors, there are different definitions are given by treaties on different level. In my opinion, that the overall conclusion shall be as long as the definition is met with one is given by international customary law then the term is well defined. Those definitions are mostly given by ECT, NAFTA, ICSID. Most of the BITs are somehow follows one definition is given by them. The BIT between Japan-Papua New Guinea follows the ECT definition. On the other hand, the test to determine whether investors and their investments fall under the specific treaty are commonly accepted and followed. 
Most of the disputes arise under the breach of Fair and Equitable Treatment. Due to its broad definition as it is regarded to cover a wide range of issues, there is no commonly accepted definition in terms of the meaning and its overlapping with other standards. Thus, it can be divided into two parts, the standard still remains unified. The analysed BIT states a prohibitory part to prevent arbitrary measures. It is still part of an ongoing debate whether the two standard shall be treated separately or the first covers the second standard. I tend to think, there is a reason that the two standard is stated separately, however under the same Article in the Japan-Papua New Guinean BIT. It may be that the treaty drafter’s intention was that there are significant overlaps between those issues but they shall be still treated as separate. The treaty is silent in its wording about discriminatory measures, which is more likely part of the FET standards due to any discrimination is unfair, unjust.
Lastly, the Most-Favoured-Nation clause was under consideration. It express a benefit between the contracting parties, to ensure that the investment of the investor is not treated less favourably than an investment of a third party. This is a commonly used and accepted clause in International Trade Law, in GATT Article I., and it makes sense that it needs to be adopted into BITs. There is a conclusion that the MFN clause only operates to the extend of the treaty. Also, in some aspects there are relations between FET and MFN. On one hand, the overlapping between standards cause hardship to the decision makers when they are facing to settle a certain despite. On the other hand, if the standards are in force would be strictly defined, and clearly distinguished, there would be gaps between legislations which likely cause uncertainty on international level in terms of investment, trade and in any economic relationships between foreign nations.