Outsorting is a set of rules that you can set up in order to control the plausibility of your billing & invoice documents.
IS-U gives you a set of standard controls most of them based on the amount of each document but you can develop different rules.
In order to outsort a document you need to complete the customizing for this, in the case of billing the customizing is: SPRO-->SAP Utilities-->Contract Billing-->Billing Execution-->Outsorting for Billing.
The outsorting for billing is assigned at contract level and for invoices it can be assigned at contract account or rate category level.