In ECS all systems are updated on the main thread. The order in which the are updated is based on a set of constraints and an optimization pass which tries to order the system in a way so that the time between scheduling a job and waiting for it is as long as possible.
The attributes to specify update order of systems are [UpdateBefore(typeof(OtherSystem))] and [UpdateAfter(typeof(OtherSystem))]. In addition to update before or after other ECS systems it is possible to update before or after different phases of the Unity PlayerLoop by using typeof(UnityEngine.Experimental.PlayerLoop.FixedUpdate) or one of the other phases in the same namespace.
The UpdateInGroup attribute will put the system in a group and the same UpdateBefore and UpdateAfter attributes can be specified on a group or with a group as the target of the before/after dependency.
To use UpdateInGroup you need to create and empty class and pass the type of that to the UpdateInGroup attribute
public class UpdateGroup
{}
[UpdateInGroup(typeof(UpdateGroup))]
class MySystem : ComponentSystem