The parameter var corresponds to an ATL variable that contains the source model element from which the searched target model element is produced. 

s.children->select(c | c.attr.oclIsUndefined())->first()

thisModule.resolveTemp(s.children->select(c | c.attr.oclIsUndefined())->first(), ...)

rule Parent2Container {
  from
    source : MMA!Parent
  to
    target : MMB!Container (
      children <- source.children->collect(c | thisModule.resolveTemp(c, 'second'))
    )
}

rule Child2MetaClassAndElement {
  from
    s : MMA!Child
  to
    t : MMB!MetaClass (...),
    second : t : MMB!Element (...)
}