What is the difference between max fill and the sweet spot?

Max fill is the upper pressure limit set by the manufacturer.

The sweet spot is the pressure range over which the gun produces its most consistent velocity, which on an unregulated PCP is usually a band somewhere below max fill. On many guns the bell curve peaks below max fill, so filling all the way to the top can briefly suppress velocity (valve lock) before it climbs as pressure drops. A chronograph and a few shot strings will reveal your specific gun's sweet spot.