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.
"Imperfective aspectual reference is commonly divided into two subtypes: progressive and habitual (e.g., Bybee et al. x000E_1994: 151_x000E). To establish that a form is an imperfective aspect, then, we need to determine that it can have both progressive meaning and habitual meaning, depending on context." (Cover & Tonhauser 2015: 324)