Triple Point of a compound and critical point Is it necessary for all compounds to have a triple point? Also, how is the triple point determined?
Suppose a substance does not have a triple point, so how do researchers agree on the impossibility of existence of a triple point in such a case? Also, can there be multiple triple points for a compound?
Also, since the vapour equilibrium curve ends at point E, what is the phase of water beyond that point?
The essential condition is the compound thermal stability.
If it decomposes below its melting point, it does not have a triple point.
If it decomposes before properties of gaseous and liquid phases converge to each other,  it has just estimated, extrapolated triple point.
Beyond CP we talk about supercritical fluid, it kind of shares many properties of both liquid and gaseous worlds.
It is usually determined experimentally, or as an extrapolation.
See the picture of liquid/gas phase system below, at and above the critical point on Wikipedia: Critical_point