In CRM I'd like to add new message subtype description when I will create quotation or confirm quotation. I'd like them to display.

I understand the workings of all the mechanisms:

def _track_subtype(self, init_values):
    self.ensure_one()
    if 'stage_id' in init_values and self.probability == 100 and self.stage_id:
        return self.env.ref('crm.mt_lead_won')
    elif 'lost_reason' in init_values and self.lost_reason:
        return self.env.ref('crm.mt_lead_lost')
    elif 'stage_id' in init_values:
        return self.env.ref('crm.mt_lead_stage')
    elif 'active' in init_values and self.active:
        return self.env.ref('crm.mt_lead_restored')
    elif 'active' in init_values and not self.active:
        return self.env.ref('crm.mt_lead_lost')
    return super(Lead, self)._track_subtype(init_values)

And view in crm\data\mail_data.xml