Hi @jvanrossum,
Could you please share a bit more about your scenario?
Do you want to add one or more predefined labels for each assignment of Assignment table?
You could consider take a try to created a Middle table called AssignmentLabel table, which has two columns: AssignmentId and LabelId. Then add AssignmentLabel table and Label table as two data sources within your PowerApps app. We could use this Middle table to store the corresponding AssignmentId and LabelId.
Please take a try with the following steps:
- Add a Custom Data Card within your EditFrom
- Wihtin the Custom DataCard, add a Label control, Text property set to "Labels". Then add a Combo box control (called "ComboBox1"), set Items property to Label table, the DisplayFields and SearchFields property sre both set to following formula:
["Name"]
The selectMultiple property set to true.
- Set the OnSelect property of the Submit Button control to the following formula:
SubmitForm(Form1);ForAll(RenameColumns(RenameColumns(ComboBox1.SelectedItems,"Name","Name1"),"Id","Id1"),Patch(AssignmentLabel table,Defaults(AssignmentLabel table),{AssignmentId:Gallery1.Selected.ID,LabelId:Id1}))


Note: The Gallery1 represents the Gallery control within your first screen.
Please check and see if the following blog would help in your scenario:
https://powerapps.microsoft.com/en-us/blog/multi-select-pattern-on-sharepoint-lists/
More details about the RenameColumns, ForAll and Patch function in PowerApps, please check the following article:
RenameColumns, ForAll and Patch function
Best regards,
Kris