Hello, if I understood correctly that you want to add a field choice only to one table and don't include it also child table, you can try one of these steps:

1. Add choice on Project table and add an override on child table (use Configure Choices and remove On Hold)
2. Add a choice on Project table by an onload client script with g_form.addOption()
   Documentation: GlideForm addOption() 

If my answer helped you, please mark it as correct and helpful, thank you 👍
Martin