what is lockedGrid(normalGrid) and where I can find it's document

I am new for extjs. When study the code : mygrid = mytree.lockedGrid

I have hard time to find out what is lockedGrid ? what is normalGrid ?

I search ExtJs Doc only find lockedGridConfig and normalGridConfig and I have to guess lockedGrid mean left panel and normlGrid mean right panel.

Can someone point me where is the document (definition ) for lockedGrid and normalGrid .

Thank.

what is lockedGrid(normalGrid) and where I can find it's document