While “which came first, the chicken or the egg?” might have proven to be one of the greatest mental conundrums of all time - the question of which came first, the “Apex or the Inpex” is a much simpler riddle to solve - with the answer being the “Apex” (or the “Ding”, if using PANArt terminology).
The ‘Ding’ refers to the central domed note found on all PANArt Hang - and is the lowest note on the instrument - it is played by either striking the protruding dome directly, or by striking the area surrounding it. Another name for this structure as used in a more general way is ‘Apex’ (meaning ‘the tip’, or ‘highest point’), and is a label believed to have been coined by Pantheon Steel (possibly to avoid legal problems with PANArt) in the creation of their own instruments.
An alternative structure of this central note can be found in the form of the ‘Inpex’ (a word that appears to have no definition prior to being used in HandPan terminology - and is also believed to have been coined by Pantheon Steel), and is essentially the exact opposite of the Apex, being instead of a protrusion, a depression into the surface of the steel.
The following two videos of a pair of Saraz HandPan tuned to the same scale, with one featuring an Apex, and the other an Inpex, demonstrate the difference perfectly, between the two approaches...