It is not uncommon to refer to the file names of actions in other modules as the file you want your action to be before or after. This creates a soft dependency on that other module - your action will still be present even if the other module is disabled (so the before or after file will be missing). If the action is removed or renamed (renaming is discouraged), your action may appear in the wrong place. As a safeguard, it is possible to specify more than one before/after attribute - in practice this is seldom done (it's not the end of the world if ordering breaks, and you usually know what modules your module will be used with, and it's unlikely that both before and after files will disappear, so the action is likely to remain generally where it's supposed to be).
If you are using NetBeans 5.0 or greater's module-building support, the new Action template will let you specify a location for the action in the wizard and generate the attributes for you.
You need to have at least one before attribute and one after attribute to guarantee the exact location of your item.
Source: NetBeans FAQ