The optional keyword is used in map construction to allow a value not to exist. Normally if an RValue→ does not exist if will trigger an error.

Syntax

Though this may technically be used any place an RValue is used it makes the most sense in a with→ construct.

set %MyVar% with
  %One% %MyOne%
  %Two% optional %MyTwo%
end

In the above code if MyOne does not exist an error will be triggered. If however MyTwo does not exist the Two key will simply not be set in the map.