A typed cell is a label when nothing reads it and an input when something does. A formula
is a link when it is one cell of another sheet (negated or scaled at most, or a reference
built with INDIRECT), a reference when it is one cell of this sheet (negated at most),
else computed where something reads it and output where nothing does. "Who reads it" is
the sheet's own formulas, so a cell only another sheet reads is a label or an output.
A finance model's color convention (inputs blue, links green, calculations black) is a
statement about this layer, which is what the styleRole check reads
(ICell.ErrorCheck).
A cell's job in the model, read from two facts: what the cell reads and who reads it.