You typically specify the text of an item's label when adding the item to the tree-view control. The TVM_INSERTITEM message includes a TV_ITEM structure that defines the item's properties, including a string containing the text of the label.
A tree-view control allocates memory for storing each item; the text of the item labels takes up a significant portion of this memory. If your application maintains a copy of the strings in the tree-view control, you can decrease the memory requirements of the control by specifying the LPSTR_TEXTCALLBACK value in the pszText member of TV_ITEM instead of passing actual strings to the tree view. Using LPSTR_TEXTCALLBACK causes the tree-view control to retrieve the text of an item's label from the parent window whenever the item needs to be redrawn. To retrieve the text, the tree-view control sends a TVN_GETDISPINFO notification message, which includes the address of a TV_DISPINFO structure. The parent window must fill the appropriate members of the included structure.