Gas Furnace Electrical Schematic Diagram - Meaning of Plus Sign inside Circle?
They're ground screws

Those are screw-head symbols (stylized Philips heads) that represent where chassis ground wires are screwed to the furnace chassis/sheet metal.  I cannot tell you why they're used for inside-the-control-box grounds though (vs. the use of the regular earth symbol for outside-the-control-box-grounds).  Perhaps it has something to do with them being explicit wire connection points?