Reverse charge

Dear Sir, Whether threshold limit of Rs. 20.0 Lacs is applicable for professional service provider (CA/CS/ICWA etc.) If a professional service provider registered under GST, but gross receipt of Rs. 20.0 Lacs does not exceed in previous year then whether they have to charge GST. If Professional registered under GST but don’t charge GST in their invoice due to exemption of threshold limit, whether RCM applicable to recipient of services.