Imagine that you have a directory full of images and you want to resize them so that they are no larger than m x n pixels.
This means that images that are already smaller than m x n should be untouched. It would be possible to modify each action plugin to make this decision but this would be very inflexible. A better way might be to allow a plugin to suppress the action of the following plugin by setting a flag.
The simplest method would be to allow the user to simply type a Python expression in a text box; Phatch would execute this expression in a local environment consisting of all the usual variables. If the result is false then the succeeding action would be suppressed.
In principle a generic plugin could be created that would allow the user to add UI elements for each of the defined variables but this would be considerably more complicated.