buffer-display-table is a variable defined in `C source code'.
Its value is

  • Automatically becomes buffer-local when set.

Display table that controls display of the contents of current buffer.

If this variable is nil, the value of `standard-display-table' is used.
Each window can have its own, overriding display table, see
`set-window-display-table' and `window-display-table'.

The display table is a char-table created with `make-display-table'.
A char-table is an array indexed by character codes. Normal array
primitives `aref' and `aset' can be used to access elements of a char-table.

Each of the char-table elements control how to display the corresponding
text character: the element at index C in the table says how to display
the character whose code is C. Each element should be a vector of
characters or nil. The value nil means display the character in the
default fashion; otherwise, the characters from the vector are delivered
to the screen instead of the original character.

For example, (aset buffer-display-table ?X [?Y]) tells Emacs
to display a capital Y instead of each X character.

In addition, a char-table has six extra slots to control the display of:

the end of a truncated screen line (extra-slot 0, a single character);
the end of a continued line (extra-slot 1, a single character);
the escape character used to display character codes in octal
(extra-slot 2, a single character);
the character used as an arrow for control characters (extra-slot 3,
a single character);
the decoration indicating the presence of invisible lines (extra-slot 4,
a vector of characters);
the character used to draw the border between side-by-side windows
(extra-slot 5, a single character).

See also the functions `display-table-slot' and `set-display-table-slot'.