First of all, yours is not a "pergunta econômica" a terse question, but a "pergunta sobre economia" a question about/on Economics (or Macro-Economics if you wish).
Now the question at hand.
There are two equations describing the Keynesian model, right?
Y = C + I + G is the first one, and then Y = C + S + T.
My question is, why the second equation is not Y = C + S + I + T?