LV2 External UI extension is an LV2 UI extension, subclass of http://lv2plug.in/ns/extensions/ui/#UI just like http://lv2plug.in/ns/extensions/ui/#GtkUI is.
It defines LV2UI_Widget pointer/handle as a pointer to LV2_External_UI_Widget, defined in the lv2_external_ui.h header (see below).

• C header file: lv2_external_ui.h
• DSSI-like universal "wrapper": dssi_wrapper.c.
  Example DSSI-like implementation of external UI (made originally for lv2 nekobee). Must be built with defines for UI_EXECUTABLE and UI_URI (strings).

This extension used to be available under the http://lv2plug.in/ns/extensions/ui#external URI, and later http://nedko.arnaudov.name/lv2/external_ui/.
These old URIs are deprecated and the extension is available under the new http://kxstudio.sf.net/ns/lv2ext/external-ui URI.
Or if you prefer, the old extensions are deprecated and a new one with exactly same semantics but different URI is available.

NOTE: The old http://nedko.arnaudov.name/lv2/external_ui/ URI has been removed from the header file, since there were no plugins using it.

List of plugins that use this extension:

• Calf (custom version as available here)
• DISTRHO Plugins and Ports
• DrumGizmo
• drumkv1
• linuxDSP
• meters.lv2
• Pianoteq
• samplv1
• sisco.lv2
• synthv1
• tuna.lv2

List of hosts that use this extension:

• Ardour
• Carla
• Jalv (using this patch)
• Qtractor