when looking at the FCOM
One should keep in mind that this stuff tends to have all sorts of terminology differences across the Industry. As a result, it is necessary to maintain good housekeeping and cite which documents are being referred to ?
there is no definition of H-arm.
Generally one would expect H-arm to be the CG position of interest .. either of the whole aircraft or a given loading item ie a variable quantity. Numerous references in the AI document are consistent with this suggestion.
where would you find the specific aircraft's H-arm
.. in the descriptive material relating to the weight and balance manual (or similar document) for the Type or the loading system for the particular tail number.
in the top left box, page 121 in the Getting to Grips document? In this example above, is H-arm = 18.93 a generic figure or is that the one to use on the manual loadsheet?
For the location cited, the H-arm of interest is the particular tail number CG for the aircraft in the dry operating weight configuration (which can vary a bit so one needs to check the definition for the particular use).
The value 18.93m, as cited, is this DOW configuration value.
Note that you must read the term in context. It might be referring to the whole aircraft CG or it might be referring to the local loading CG position for a particular load item going into the aircraft.