It is extremely rare that you find any two baseball fields the same. It can be something as small as a sprinkler head that broke, to a railroad track that crosses through right field. So before every game the umpire will meet with the two team managers (or captains), exchange lineup cards and discuss the Ground Rules. Ground Rules are not the same as general Baseball rules. Ground Rules cover things unique to the field you are getting ready to play on. This is where the term, “Ground Rule Double” originated. Along with things like what constitutes a Home Run based on this field’s particular fence. Or how does a train passing through right field affect the game.
This photo was taken at the Ft Lupton, 2015, Annual Heritage Fair, where two teams from the Colorado Vintage Base Ball Association played, the Central City Stars vs the Rio Grande Reds. Here umpire Bruce Foster goes over the Ground Rules with the respective team captains.