Item &1 is assigned; changing &2 will unassign the item partially

What causes this issue?

The material is relevant to Supply Assignment (ARun) and an assignment
exists for this document item.
The document item will be unassigned for one of the following reasons
and changing the document item will partially unassign the item.
o,,You have defined the Customization for the document item under
<LS>Cross-Application Components > Advanced Available-to-Promise (aATP)
> Supply Assignment (ARun) > Cross-Applications > Purchase Order
Handling</>, changing the document item will unassign the order. You
have set the field check as <ZK>W = Warnings Before an Order is
Unassigned</>.
o,,You have not defined a rule to unassign orders under
<LS>Cross-Application Components > Advanced Available-to-Promise (aATP)
> Supply Assignment (ARun) > Cross-Applications > Purchase Order
Handling</>. The item will be unassigned automatically.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Define rules to unassign orders for an application type in Customizing
under <LS>Cross-Application Components > Advanced Available-to-Promise
(aATP) > Supply Assignment (ARun) > Cross-Applications</>.
By defining a rule to unassign orders, you specify exactly how the
system should react when an application type is modified (for example,
change in requirement date, goods receipt date, plant, storage location,
and so on).
You set the appropriate fields listed in the application type in
Customizing. For field checks, you can assign one of the following
options.
N = No Action
W = Warnings Before an Order is Unassigned
E = Change Not Permitted
Depending on the selected parameter, the system decides how and when the
quantities should be unassigned.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.