This was something I had a look at recently. I had to hide certain dimension for specific security roles.
I won’t go into the details of why as that is another longer conversation.
This can be achieved using XDS (security policy).
One thing you have to keep in mind is if a dimension is defaulted it will contain a value.
I set up a vendor with a default dimension as per screenshot.
On the form it showed the 3 dimensions but in the database it was actually defaulting as above full 5 dimensions. This is because it is defaulting the RecId reference to the dimension value combination.