Let's say we have a SKU in Zentail called SKU-Hat.  Here are some item specs for the product in Zentail.

This is how we would expect the same product to be represented in Seller Central:

In other words, the original SKU-Hat represents only FBA inventory, while any merchant fulfilled inventory will be shown only on SKU-Hat+MF+.  This allows sellers to take advantage of several features like autoflip FBA to MF, FBA Price Overrides, and more.

Now, let's say that restock FBA with 5 units.  In Zentail it will show this:

There are two ways this could show on Amazon Seller Central now.

This means we show only the FBA quantity until it runs out.  So despite having 10 units in your main warehouse this is how it will be displayed:

And once FBA runs out of stock, the listing will make the merchant fulfilled inventory available:

This means both FBA and Merchant Fulfilled quantity will be available for purchase at the same time.  On Amazon it will show like this: