An imperfective marker is a verbal marker that can indicate (= is compatible with) ongoing aspectual reference and habitual aspectual reference but that is incompatible with bounded aspectual reference.
According to this definition, a purely habitual marker is not a subtype of imperfective marker, and a purely progressive marker is not a subtype either. Imperfective markers must be like the Russian and French imperfective constructions and occur both with ongoing and habitual reference.