Addon or Hacks and Modifications

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What is the difference between both?
When is an addon "an addon" and when becomes a "Hacks and Modifications"?

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The Pixelpost framework has addon support built in.
Basically, any PHP code you write in an addon file (in the addon folder) will be executed at "runtime". Your addons can affect any of the variable used by the main Pixelpost codebase. You can even change things already executed, for example, my Short Notes addon changes text already worked on by Pixelpost.
A hack is a direct modification to the Pixelpost code. For example, adding an option to the existing administration pages currently requires a hack.

Addons are self contained php files, which you put in the addons folder, and are executed the way Pixelpost is designed to work.
Hacks are modifications to the standard Pixelpost sourcecode.