With setting 'Backorders allowed: YES' (not the default setting):
First of all, to set the 'backorders allowed' to YES, follow these steps:
1. Click in left menu on 'Settings'.
2. Now go to the top of the page where the salesprofile settings are. Click in your salesprofile on the 'i' icon ("general info").
3. On the setting page that comes up, scroll down to almost the bottom, you will find the option 'Backorders allowed (yes/no').
Implications of setting the backorders to yes:
1. When you, as a merchant, never set any inventory or stock level on products or their variants then there is no impact. People will be able to order as much as they want. As a merchant, since you didn't set an inventory, you can deliver as much as you want.
2. When you, as a merchant, always set an inventory or stock level on every product or their variants, then there is an impact. Meaning, people will be able to order the number of stock items they would like to have, even when this is more than you have in stock.
Again, offering backorders is a commercial decision for you as a business. Your customers expect a soon delivery so in case they ordered items that are not in stock, but you don't want to lose sales, communication is king. It is important to note what backorder is-you are basically asking your clients to pay in advance for a commodity. They will of course be concerned and expect daily updates. So, contact in time is absolutely important!