Pretty recent feature of enabling external access to emacs and org. From a 2019-09-27 point of view.

In the manual for org version 8.2.9, which comes with my installed org version 9.1.6, pointing to an elpa folder 9.0.9. there is just a short paragraph about the protocols.

"You can set up org for handling protocol calls from outside applications that are passed to emacs through emacsserver. For example, you

• can configure bookmarks in your web browser to send a link to the current page to org and create a note from it using capture. Or you
• could create a bookmark that will tell emacs to open the local source file of a remote website you are looking at with the browser." 9.4 Protocols for external access of the org manual, version 8.2.9

But it points to a worg tutorial, which in turn leads to the configurational requirements of the protocols feature:

• the setup of the operation system, i.e., in linux, the gnome or kde server registry
• for firefox the MozillaZine's Register protocol entry.

The 9.2 version of the manual expands on these requirements and offers three predefined protocols called, with increasing complexity of the invocation:

• store-link – Store a link, push URL to kill-ring.
• capture – Fill a buffer with external information.
• open-source – Edit published contents.

The setup for gnome on ubuntu is to

• install gconf2 which provides the command =gconftool-2=and
• put two url-handlers/org-protocol entries into the 📂 /desktop/gnome section.
• In the example below we also check the entries. I could also look them up in the xml file 📂 .gconf/desktop/gnome/url-handlers/org-protocol/%gconf.xml of my home directory.

gconf "is a system for storing application preferences" (→ gnome.org) and "similar to the Windows Registry" (→ enWiki). See also GConf: Manageable User Preferences [Pennington2002].

sudo apt install gconf2
gconftool-2 -s /desktop/gnome/url-handlers/org-protocol/command '/usr/bin/emacsclient %s' --type String
gconftool-2 -s /desktop/gnome/url-handlers/org-protocol/enabled --type Boolean true
gconftool-2 -R /desktop/gnome/url-handlers