Main Tank: A separate, main fuel tank for supplying fuel to the engine or a day tank.
Day Tank: A fuel tank, located inside a structure that provides fuel to the engine.
Integral Tank: A fuel tank furnished by the generator manufacturer and mounted on the engine.
Base Tank: A fuel tank furnished by the generator manufacturer and mounted under the engine as a base.

Why use a Day Tank between the Main Tank and the generator?

Paragraphs 7.9.2 and 7.9.2.1 of NFPA 110 state that:

7.9.2. Fuel tanks shall comply with one of the following:

(1) Be close enough to the energy converter for the fuel lift (i.e. suction head) of the energy converter fuel pump to meet the fuel system requirements.

(2) Be provided with a fuel transfer pump and day tank.

7.9.2.1. If the engine manufacturer’s fuel pump static head pressure limits are exceeded when the level
of fuel in the tank is at maximum, a day tank shall be utilized.

Based on NFPA 110 – Standard for Emergency and Standby Power Systems, 2025 Edition
National Fire Protection Association