Entity relationships define how records can be related to each other in the database. ​

A 1: N relationship is used when one record of primary entity can be associated to many other related entity records because of a lookup field on the related entity.

For example, consider an entity plan (custom entity) which has multiple records like montlyplan1, monthlyplan2, monthlyplan3. If a contact wants to select one among  the above plans, there must be a lookup on contact entity and there must be a sub grid on plan entity. 

Let us create a 1: N relationship between contact and plan in the new UI.

Note:

Make sure you create a custom entity plan and add a field plan in the contact entity to proceed further.

Open  make.powerapps.com.

Open the entity plan.

Open relationships.

Select “Add relationship”. Select “one to many”

Choose the entity with which you want to create one-to-many relationships.

 In this case its contact, add the attribute to which to want to look up in contact(plan).

Open your dynamics 365 account and go to plans. Create few records for plans.

Go to contact entity and go to plan attribute, you will find lookups created on plan.

Select a plan which you prefer.

Open monthlyplan1 record in plan entity and you can find subgrid of contacts who choosed this record.