In Routine§

See primary documentation in context for trait is default

multi trait_mod:<is>(Routine:D $r, :$default!)

There is a special trait for Routines called is default. This trait is designed as a way to disambiguate multi calls that would normally throw an error because the compiler would not know which one to use. This means that given the following two Routines, the one with the is default trait will be called.

multi f() is default { say "Hello there" }
multi f() { say "Hello friend" }
f();   # OUTPUT: «"Hello there"␤»

The is default trait can become very useful for debugging and other uses but keep in mind that it will only resolve an ambiguous dispatch between two Routines of the same precedence. If one of the Routines is narrower than another, then that one will be called. For example:

multi f() is default { say "Hello there" }
multi f(:$greet) { say "Hello " ~ $greet }
f();   # "Use of uninitialized value $greet..." 

In this example, the multi without is default was called because it was actually narrower than the Sub with it.