As described in the comment of PHPBB-17359, it would be clearer to rename the section of available but not installed extensions to "not installed" (quote from the reply to the ticket by LukeWCS ):

Hello

This is basically a good change that could help administrators to recognize that a non-installed extension is not the same as a deactivated extension. The fact that there has not been an explicit separation of these two groups is partly responsible for the fact that administrators repeatedly handle extensions incorrectly, for example simply deleting an extension without uninstalling it first, which can then lead to the forum being blocked depending on how an extension works. And depending on the extension, deleting the cache is not enough.

I would therefore like to suggest changing the text of the new language variable and renaming the language variable and template array.

• Name of the template array: NOT_INSTALLED
• Name of the language variable: EXTENSIONS_NOT_INSTALLED
• Text of the language variable: "Not installed Extensions"

For this reason (strict separation) I have explicitly inserted a visual separation in Extension Manager Plus from the very beginning (1.0.0), see screen. This screen was created on one of my local test boards, in this case a phpBB installation, where I am specifically testing the change from 3.3.14-rc1 with EMP 2.1.0.

The basis of my suggestion are the primary extension manager functions of phpBB:
 

phpbb\extension\manager->all_available()

phpbb\extension\manager->all_configured()

phpbb\extension\manager->all_enabled()

phpbb\extension\manager->all_disabled()

all_available() are all extensions that are present in the ext folder AND are valid. This function therefore has a completely different meaning, which is why the name EXTENSIONS_AVAILABLE of the new language variable is not ideal. As you can see in the EMP screen, I use also "Available extensions" in the info table at the top, which is directly related to the all_available() function.

But it is clear to me why this new language variable was named this way; in principle the name is consistent, since the underlying controller function was named list_available_exts(). Therefore, renaming this function would be an option to make everything fit together: list_not_installed_exts(). Since this is a controller function of the phpBB Extensions Manager, it is highly unlikely that an extension uses this function.

The two primary arguments for my suggestion are:

• Clearly communicate that an extension is being "installed" and not just "activated". That's why the section should also be titled clearly and unambiguously.
• Consistent identifiers (code) and texts (UI).