you may be confusing two items
in old church yards before the 1881 burial act that required appointment of s extons there were no plots except in some newer yards such as Douglas St Georges, Braddan new etc, prior to this there were conventions as to which family 'owned' ie generally buried their dead in certain areas (there are several disputes mentioned in Manxnotebook for early 18th C as these are often found among the wills) - what is recorded by the IoMFHS is the surviving MI and the IoMFHS designation (printed at the start of each book of MIs) is just a locator as to position and has no official use.

In newer yards opened post the burial act there will generally be a plot/grave number and this is often recorded in the burial register thus even if there is no MI, those buried in the same plot can be found. The museum have ownership books for some yards eg Braddan New (the older sections) that show who owned the plot (not always the person buried there)