These are like a temporary variable storage for matrixes.
The matrix has the facility to post to other widgets selections made on the matrix. This is useful for knowing which area is currently selected, what cell is currently selected, and what text is currently under your selection.
This function overwrights the contents of another widget outside of the matrix, which can be hidden from the user with visible=false. So then you can create triggers on these additional widgets, which can be used to control the actions of your programme. So if a user clicks on a populated cell, you can then have a specific trigger that actions that click and knows what the user just clicked on. Likewise it is used for adding new content to the matrix grid by knowing where the user current drag selection is.
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