Extension PointExtension-Points are similar to targets in that they have a name and a depends list and can be executed from the command line. Just like targets they represent a state during the build process.
Unlike targets they don't contain any tasks, their main purpose is to collect targets that contribute to the desired state in their depends list.
Targets can add themselves to an extension-points's depends list via their extensionOf attribute. The targets that add themselves will be added after the targets of the explicit depends-attribute of the extension-point, if multiple targets add themselves, their relative order is not defined.
The main purpose of an extension-point is to act as an extension point for build files designed to be imported. In the imported file an extension-point defines a state that must be reached and targets from other build files can join the depends list of said extension-point in order to contribute to that state.
For example your imported build file may need to compile code, it might look like:
<target name="create-directory-layout">Call-Graph: create-directory-layout --> 'empty slot' --> compile
<target name="compile" depends="ready-to-compile">
And you need to generate some source before compilation, then in your main build file you may use something like
<target name="generate-sources"Call-Graph: create-directory-layout --> generate-sources --> compile
This will ensure that the generate-sources target is executed before the compile target.
Don't rely on the order of the depends list, if generate-sources depends on create-directory-layout then it must explicitly depend on it via its own depends attribute.