The OCL Operator Default is an operator of the static class Encoding.

Default uses the default encoding for the running .Net implementation.

Encoding.Default.GetBytes(Customer.allinstances->first.Name)

This will return a Blob (Byte Array). Specifically, it takes the name of the first customer in your database and translates it into the raw numeric values (bytes) that represent those characters according to your operating system's current local encoding