I update Adobe Flash every time that an update is available. The Flash preference pane says:

NPAPI Plug-in version 15.0.0.239 is installed.
PPAPI Plug-in is not installed.

There have been similar messages for previous versions. Flash seems to behave normally on those sites that require it but I don't understand what the above message means. Thoughts?

NPAPI and PPAPI refer to a particular architecture used (or not, as the case may be) by your browser(s) for the Flash plugin. As you only need one, the lack of the other isn't an issue.

Thanks!

To add a bit more information:

There are two commonly-used schemes to write plugins for Web browsers: NPAPI (the plugin scheme invented by Netscape) and PPAPI (the plugin scheme invented by Google for Google Chrome). Most browsers use NPAPI.

Some versions of Chrome can use NPAPI or PPAPI plugins, but you don't want to install the same plugin twice else you risk breaking things in weird and interesting ways. Google announced a while back that they would be discontinuing support for NPAPI plugins; I don't know if the current version of Chrome can use them or not.