Default POS Customer lets you choose a customer for unidentified ShopifyPOS sales.
ShopifyPOS lets you choose Shopify customers or add information like email address. BigBusiness can match them to Customers in BigBusiness using their email addresses.
If a customer is not identified you may want a special Customer in BigBusiness to be used. Note this is not the Default Customer for Shopify but special for ShopifyPOS.