Composed with ... by these characters

[Eli Zaretskii]

  emacs -Q -nw

After "C-h H", go to the "Bengali" line, type C-f to get to one of the
non-ASCII characters, and type "C-u C-x =".  In the "*Help*" buffer,
you will see the following snippet, among other text:

    Composed with the surrounding characters "?" and "??" by these characters:
     ? (#x2476)
     ? (#x2494)
     ? (#x2434)
     ? (#x2482)

What does it mean "composed with ... characters ... BY THESE
CHARACTERS?  This probably needs to be rephrased, but how?

Also, is it normal that on a tty such composed characters show more ?
characters than glyphs on a GUI display?  For example, in a GUI
session, the word inside parentheses after "Bengali" is displayed with
2 glyphs, while a tty shows no less than 5 question marks.  Is this
expected behavior?