What does 'onClassExtended' do when defining a class?

[gui0506]

Ext.define('Algo.data.Simulated',{    
    requires:[//....],    
    onClassExtended:function(cls, data){// ....}
})

Here is the code. I cannot find any documentation about it on Sencha's website. Also, what do cls and data mean?

[evant]

Seems like you already asked here and got a pretty good answer: http://stackoverflow.com/questions/3...fining-a-class