is app store credit balance different than a apple id balance ?

The name App ID balance was changed some time ago to Apple Account balance.

The App Store Credit is the same as Apple Account balance.

My default payment method is my Apple Account balance, Apple uses the name Store Credit on the invoice.

For example:

Another example is:

Here's how long it takes for a refund to appear on your account or statement:

Store credit – It may take up to 48 hours to see the refund in your Apple Account balance.

See -> Check your Apple Account balance

Check your Apple Account balance – Apple Support (UK)

or 

Tap App Store -> tap your Profile (top right of the screen) -> then you will see your Apple Account balance.

I don’t think this really answers the OP’s question as I’m having what seems to be the same issue.

I had a refund approved a week ago and there was still no change to the balance that appears in the App Store account page (where I most frequently see my balance when reviewing app updates).

After searching for info on the refund process I then viewed the Account Settings page & discovered…

So the credit has been applied but for some reason it’s not showing on the main Account page.

So then my question becomes when the smaller of the two values is exceeded, what happens then ? The account page will show a negative balance, or does it just need a new transaction to adjust, or will any subscriptions be denied because one source thinks the balance is lesser than what it actually is ?!? 🤷‍♂️🤔

That depends exactly what you are looking at.

Where exactly are you seeing this 0, that is different to the App Store balance?

Can you provide a screen shot? Redact any personal information.