Kubernetes ReplicaSet another of manifest file

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: rs-nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx 
  template:
    metadata:
      name: nginx
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx-container
        image: nginx

The main difference from ReplicationController is that ReplicaSet requires a selector. ReplicaSet can manage pods that are not created during ReplicaSet creation: for example there are a pods created before creation of ReplicaSet.