Our usual definition is:
“Use” is the power/energy consumed by the house. It is positive by definition.
“Generation ( / Solar)” (usually PV but it could be wind or hydro) is the power/energy from the PV etc infeed. Usually positive during daylight, it can be negative at night because the inverter does consume a small amount of ‘keep-alive’ power.
“Grid” is the power/energy that flows in the connection from the supply authority. We define it as positive when power/energy is imported (Use > PV) and negative when power/energy is exported (PV > Use).
In your case, we would regard “Grid” as the nett sum over both legs of the supply - but bearing in mind what Bill Thomson wrote elsewhere, your meter might charge you for importing on one leg and charge you for exporting on the other leg.