SharedBetweenAnimatorsAttribute is an attribute that specify that this StateMachineBehaviour should be instantiate only once and shared among all Animator instance. This attribute reduce the memory footprint for each controller instance.
It's up to the programmer to choose which StateMachineBehaviour could use this attribute. Be aware that if your StateMachineBehaviour change some member variable it will affect all other Animator instance using it. See Also: StateMachineBehaviour class.
using UnityEngine;
[SharedBetweenAnimators]
public class AttackBehaviour : StateMachineBehaviour
{
public void OnStateEnter(Animator animator, AnimatorStateInfo stateInfo, int layerIndex)
{
Debug.Log("OnStateEnter");
}
}