sig Dog {}
-- "Beauty without Vanity, Courage without Ferosity, Strength without Insolence"
one sig Boatswain extends Dog {}

If you'd asked Lord Byron, there was only one Boatswain---whose tomb was famously larger than Lord Byron's own.

abstract sig Student {}
sig Undergrad, Grad extends Student {}

In this example, any Student must be either an Undergrad or Grad student.

Lone sigs aren't used much; you can think of them in the same way you'd use a one sig, but with the possibility that the sig will be empty. This can sometimes be useful for efficiency. E.g., if we were modeling soup recipes:

abstract sig Ingredient {}
lone sig Potatoes extends Ingredient {}
lone sig Carrots extends Ingredient {}
lone sig Celery extends Ingredient {}
lone sig Water extends Ingredient {}
// ...

There might be dozens of possible ingredients. But if we only want to use a few at a time, it can be useful to set a lower bound on Ingredient and allow un-used ingredients to simply not exist.