Standard is the mode you are in until you apply another one. It adds nothing on top of what your profile already decides.
What it does
What you can track
Everything that is not tied to a mode: food, water, weight, blood pressure, blood glucose, exercise, symptoms and fasts. On the web these live under Track; on the phone under the Track tab.
Standard has no tracker of its own on the phone. Its mode screen lists what the mode covers and points to those logs.
When to stay on Standard
Returning to Standard
On the web, open Diet Modes and apply another mode; there is no Standard card there, because Standard is what you have when no other mode is applied. On the phone, Standard is a card under Available Modes and you can tap it to go back. See Switching Between Modes.