Pitcher warming up question

[stmaryrams]

FED rules, I've now seen three interpretations of this sitch. Players warming up on sidelines in foul territory beyond the infield while game is in progress. Similar to a major league bullpen in some parks. We've had umpires require all players to be wearing a helmet, one player protecting the other two required to and not required to wear a helmet.

There must be some kind of rule regarding this. Would always rather err on the side of safety. No problems with players warming up outside the fence.