Choices([@'New Server Requests'].'RAID_x0020_Level_x0020_1')

Choices([@'New Server Requests'].'RAID_x0020_Level_x0020_1')
Choices([@'New Server Requests'].'RAID_x0020_Level_x0020_1') includes both predefined and user-filled choices. To exclude fill-in values, you need to manually filter the options.Set the Items property of the ComboBox to only include the predefined choices:
Filter(
Choices([@'New Server Requests'].'RAID_x0020_Level_x0020_1'),
Value in ["RAID-1", "RAID-5", "RAID-10"]
)
If you don’t want to hardcode the choices, check if SharePoint’s predefined options have a specific property:
​​​​​​​Filter(
Choices([@'New Server Requests'].'RAID_x0020_Level_x0020_1'),
Not(IsBlank(Value)) && Not(Value in Distinct('New Server Requests', 'RAID Level 1'))
)
I hope this helps you to understand better how Choices functions works,
Regards.