"I am on the train" as they say( in a loud voice) on the way back from BETT. I also like the term "add on" very much. I'll take a look in the docs too and see if I can spot any changes that need doing.
New name for contributed plugins: Add-ons
So, to get this right
A moodle 'plug-in' is a 'plug-in' because it's included in the standard download and a non standard download 'plug-in' is a 'add-on' until it becomes a part of the standard download when it becomes an 'plug-in'. But I've been told on chat that 'https://moodle.org/plugins/' will stay as 'https://moodle.org/plugins/' as it says 'WELCOME to our Plugins directory of add-ons for Moodle!' but does it contain any standard download 'plug-ins'? If not then it should be the 'add-ons' directory to avoid user confusion and ease documentation / training etc. ambiguity?
I think I like the name 'add-on' as 'plug-in' depicts something that you shove into the wall and flick a switch to turn on. And it's more familiar to users as things they want as extras to their new product.
The issue here is that an Add-on on (at a around say moodle 2.0) can possibly become included as a standard plugin and distributed with core in a future moodle version (say moodle 2.5).
When that happens a generic reference to the plugin across moodle version will be 'plugin' as it was add-on for some moodle versions but standard for some. They can also in future possibly be removed from the standard distribution. I think this is the reason why its still called the Plugins directory.
Hm, perhaps the welcome message could be made a simpler or clearer.
Alternatively, we could add further explanatory text:
Standard plugins are not generally included in the plugins directory, apart from any which started life as an add-on and were then added as a standard plugin to a later version of Moodle.
(The book module is an example of this.)
So, using the 'Book' and 'Drag and Drop' as examples where their timeline would have been as an 'Add-on' for several releases and available in the database. Then when it is made a part of the standard distribution it is still an 'Add-on' and the older releases are in the database. Then a notice is placed on it's entry that since release X that it is included in the standard distribution. An example of this happening is Davo Smith's drag and drop - https://moodle.org/plugins/view.php?plugin=block_dndupload - where he states "It should not be installed on a Moodle 2.3 site as it will clash with the built-in drag and drop upload.".
Therefore to avoid confusion how about renaming everything that is a plugin, standard distribution or 'contrib' to being an 'Add-on'. Thereby keeping it simple and easy for users to remember and say 'Moodle is great as it is so intuitive' - less is more
I think this solves Aparup's described situation too. And as 'Add-ons' are listed on the 'Plugin's' adminstration menu regardless of where they came from.