Our left-side brains are seduced by quantitative scales (such as Sheldon - the American system), as if they somehow represent greater certainty than qualitative grades (such as extra fine etc of the British system). This is a pure mirage, but a good one and it is what the PCGS and NGC make money from. A fine business model!
In reality, all grading is qualitative - and all qualitative judgments are (to a degree) subjective and error prone.
In my view, TPG provides a useful rule of thumb to purchase at a distance; no better nor worse than qualitative grading provided by a reputable auction house. Neither is a substitute for the expanded qualitative descriptions and exchange of observations as per the thread above. None of this is a substitute for the subjective feel of a coin in hand - preferably broken out of it’s slab with a hammer and victory dance.