Exert refers to the inherent effect some Warcries apply to an attack or several subsequent attacks in order to make it Exerted once the Warcry has been activated. Attacks can be Exerted by multiple Warcries and receive the benefits of each so long as their type matches the type a Warcry supports.
The number of Exerted attacks remaining is shown on the Skill bar icon.
Channelling skills cannot be exerted.
Vaal skills cannot be exerted, except for Vaal Ground Slam which explicitly states an exception.
Multistrike and Exerted attacks: Multistrike Support supported skills cannot be exerted, although skills that inherently strikes multiple times, like Double Strike, consume one exerted attack per use of the skill.