It's only applicable to Job works as per definition under GST. As
“job work means any treatment or process undertaken by a person on goods belonging to another registered person and the expression job worker shall be construed accordingly” [Section 2(68)]
Job works charges As per Notification 20/2019-CT(R) Dt: 30/09/2019.
i) For diamond related services
Rate of GST reduced from 5% to 1.5% on supply of job work services in relation to diamonds.
ii) For engineering related services other than bus body bldg.
The rate of GST reduced from 18% to 12% on supply of machine job work such as in engineering industry, except supply of job work in relation to bus body building which would remain at 18%.