Question

When using M-/, the text in the current buffer is autocompleted with suggestions out of all active buffers.

Is there a way to limit the suggestions to only one specific buffer?

Answer 1

Assuming you are talking about dabbrev-expand (M-/ is the usual binding), then there are a variety of options depending on your requirements.

To search only a specific white list of buffers, the simplest approach is to set the variable dabbrev-search-these-buffers-only:

  "If non-nil, a list of buffers which dabbrev should search.
If this variable is non-nil, dabbrev will only look in these buffers.
It will not even look in the current buffer if it is not a member of
this list."

Here’s an example from a custom mode of mine (I rebind M-/ to this function for this mode)

(defun tks-dabbrev-expand (arg)
  "Expand either aliases or descriptions, depending on context."
  (interactive "*P")
  (let* ((candidates
          (if (looking-back "^\\S-+")
              " *tks-aliases*"
            " *tks-descriptions*"))
         (dabbrev-search-these-buffers-only (list (get-buffer candidates))))
    (dabbrev-expand arg)))

Note that there are several other ways in which you can filter the list of buffers which dabbrev will search inside. The dabbrev customize group has the details:
M-x customize-group RET dabbrev RET