The term jusqu’à is French for "as far as" or "up to". In a postal context, these markings were used to indicate the point at which airmail service ended for a particular piece of mail.
The book includes numerous illustrations and measurements, making it the primary tool for authenticating rare markings or identifying common ones. Jusqu-a Airmail Markings- A Study Ian McQueen
Here is why McQueen’s work remains the gold standard: The term jusqu’à is French for "as far as" or "up to"