How the Demand class works to steal the on-stock based on the Demand Class to Sales Order?

We want to use the 'Demand Class' functionality for ATP scheduling and reservation during the Sales Order booking, But we are not able to achieve below requirement:

3 different customer requesting for the same product item on the same day and we want to reserve the on-hand stock based on the 'Demand Class' set at each sales order or sales order lines.

For Example:

On-Hand: 140 EA

Cust1: 20 EA      Demand Class-Low,           Request Date: 10-Jan-2019

Cust2: 100 EA    Demand Class-High,          Request Date: 10-Jan-2019

Cust3: 30 EA      Demand Class-Medium,     Request Date: 10-Jan-2019

Expectation:

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Cust2 with High should be reserved with 100 first, then

Cust3 with Medium should be reserved with 30 second,