buffer-invisibility-spec is a variable defined in `C source code'.
Its value is

global value is the same.
  • Automatically becomes buffer-local when set.

Invisibility spec of this buffer.
The default is t, which means that text is invisible if it has a non-nil
`invisible' property.
This variable can also be a list. The list can have two kinds of elements:
`ATOM' and `(ATOM . ELLIPSIS)'. A text character is invisible if its
`invisible' property is `ATOM', or has an `invisible' property that is a list
that contains `ATOM'.
If the `(ATOM . ELLIPSIS)' form is used, and `ELLIPSIS' is non-nil, an
ellipsis will be displayed after the invisible characters.
Setting this variable is very fast, much faster than scanning all the text in
the buffer looking for properties to change.