Spark Shipping allows you to set inventory quantities boundaries within your inventory settings from different vendors/ drop shippers.
The Maximum quantity is especially helpful when you don't want to exceed a certain number of listings in your store, like eBay for example. What it does is that it sets the quantity of products mapped to the vendor to not exceed this max quantity.
The Minimum quantity on the other hand tells Spark Shipping this is the out of stock threshold and that it should set all products with a quantity below it to Out of Stock. So if you set the minimum quantity to 5 for example, Spark Shipping will set all products with a quantity below 5 as out of stock products, this is especially helpful to avoid over-selling a product that sells quickly.