Notes in Parenthesis, how are the played?

[i_am_joey_jo]

I am playing Scarlatti Kk.1 but I'm sure this appears in other pieces as well.

The quotations are around a note and I have seen this happen where the note played by the left and the right is the same making one of them redundant. (Invention 8 - Bach) but this is different.  The note on the left hand is not the same as the note on the right hand and watching youtube videos have seen it left out and also played.

What do we do with these quotated notes and why are they so?  Are the inferred?  I use the URTEXT unchanged Scarlatti version and it's in brackets... so.... what did he mean by this?