xpBranch is a modifier which produces something akin to a plant structure.
It has a stem, which has branches coming from it and these branches may themselves have branches and so on.
The number, position, length, rotation around the stem, etc. may all be set, procedurally.
The xpBranch modifier itself only produces data used for the stem of the branched object.
To generate actual branches you need a Sub-Branch to be added to it, as a child object.
This can easily be done by clicking the Add Branch Object button.
The Sub-Branch object has a similar button, which you can click to add a sub-branch to the branch.
There is no limit to how many levels are generated but usually from one to three is all that is required.
xpBranch General tab.
Checking this box activates xpBranch.
Set at Independent, by default.
You can change this to Action-Controlled.
In this mode, particles will be affected if they come into the field of effect of the modifier.
When in the Action-Controlled Mode setting, the modifier will only act on a particle when told to do so by an action.