Index entry for Object Replacement Character

[Eli Zaretskii]

I don't understand why you added an index entry for this.  Can you
explain?

Here's my take on this: Index entries exist to make it easier for the
reader to find the description/discussion of a subject.  In this case,
the subject of the text where the Object Replacement Character is
reordering of display properties in bidirectional text.  The fact that
the Emacs implementation treats text covered by a display property as
if it were a single character with the bidirectional properties of
u+FFFC is an implementation detail, mentioned by the way for those who
are interested enough to look up that character.  The text does not
explain anything about that character, and doesn't say why it was
chosen for this role.  The character is mentioned once, and that's it.

Therefore, I find it hard to believe that any reader will thing about
this character's name as a good string to try at the prompt of the
Info's 'i' command.  Likewise, I see no reason why someone who wants
to know about this character will look for its description in the ELisp
manual.  So having this index entry there is not useful.

More generally, not every term you see in a manual deserves an index
entry.  It only does deserve an entry if (a) the term is described and
explained at that place, and (b) there's a high probability that
readers will think about the phrase you put in an index entry when
they are looking for the subject described at that place.