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What do you mean that you want to "get the value of the machine selected"? What do you want to do with that value?
You may be able to make a direct reference:
- Me.zfrmStatus.Machine_Name
but that may likely refer to the first record in the subform, since your focus has moved to the Main Form.
Depending on what the "Cambiar" button does, you may want to switch the subform to a Continuous Form (instead of datasheet). This allows you numerous advantages.
First, you can resize and arrange columns more easily. Second, you can add a button in the detail section of the subform (Cambiar) and when you click on the button associated with that record, you can refer directly to that particular record:
Again, it depends on how you want to use that value.