XL stands for Extra Load. These tyres are an evolution of the
Reinforced tyre, and have been developed for heavier, higher
performance vehicles (rather than vehicles carrying big loads),
which allow a lower profile tyre with extra load carrying
capability. These tyres are constructed to cope with the additional
weight by providing higher rigidity and enhanced road holding.
Extra load tyres have a higher inflation pressure that standard
load tyres, and often have XL moulded on the sidewall, eg: