What are the keys contained in request.POST.has_key(*...*)?

Question

I was going through source code of one of the django applications in my organisation. I can see response_add method overridden in admin.py as below :

def response_change(self, request, obj):
    if '_upload' in request.POST:
         url = reverse('admin:%s_%s_change' % (
          self.model._meta.app_label,
          self.model._meta.model_name), args=[obj.pk],
          current_app=self.admin_site.name)
         return HttpResponseRedirect(url)
    else:
         return super(DfInpAdmin, self).response_change(request, obj)

I cant find anywhere on internet what is _upload here. ? Instead I see many code snippets with if request.POST.has_key("_popup"):.

What are these _upload, _popup, _continue ? Are they django provided or custom defined in some other .py of my source code?