How do calculated fields or text you add in a Form/Report control's "Control Source" property not also become new fields in the Table you built it off of   if that s kind of part of the Control Source s definition...?

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Reply from Alex Hedley:

See this MS Article

Setting | Description
A field name
The control is bound to a field in a table, query, or SQL statement. Data from the field is displayed in the control. Changes to the data inside the control change the corresponding data in the field. (To make the control read-only, set the Locked property to Yes.) If you click a control bound to a field that has a Hyperlink data type, you jump to the destination specified in the hyperlink address.

An expression
The control displays data generated by an expression. This data can be changed by the user but isn't saved in the database.

Since you use the "=" for an Expression Access then knows it's not a BOUND Column which saves data back to your Table.