Explanation for Default stock and Salable Qty
Now magento has introduced MSI feature by-default in magento.
In this you can use Multi-source inventory.
First there is a need to understand three terms:-
1.Source:- You can relate this with warehouses.
As you can have single warehouse or multi-warehouses for your website similarly you can have single source for single warehouse or multi-source for multi warehouses.
For eg. If you are selling products from multi-location like Florida US, California US etc then you will need to create sources for each inventory location.
2.Stock:- It is basically the interface which connects your source to your website.
Main thing here is that you can have multiple stocks for single source but you can’t have multiple stocks for single website.
3.Channel:- You can relate it to the website.
Source -> Stock ->Website
Here you can have one to many relationship between source and stock but you have one to one relationship between stock and website.
Then came the terms Qty and Scalable Qty.
Suppose you have a product whose Qty is 10 and customer purchase 2 products from that.
Then your qty still shows 10 but your scalable qty would be 8.
Means your qty get decreases when your product is shipped and invoiced but your scalable qty decreases as soon as you ordered the product from website.
You can refer to this https://www.atwix.com/magento-2/multi-source-inventory-in-magento-2-3/