The best field is the AI's own result for the level. A proved one is the exact maximum; the rest are the best the AI found.
Click grass to place a fence · click a fence to take it back · H hint · R restart
Paint ponds and grass, drop as many horses as you like (every one must be penned), set a fence budget, then let the AI decide the best total field.
The seed fills the ponds and leaves your horse(s) exactly where they are — paint over it to taste.
Paddocks other people built. Each one carries the AI's best field for it.