Phra Parama Sareerikathad" (Sarīrika Dhātu) in Thailand refers to relics which originated from the Lord Buddha,this term is not use for the relics of his disciples.
"Phra Thad" (Dhātu) refers to the bone or any part of the body such as hair, nail, teeth, skin, blood, etc. which are different to those of ordinary people. These relics resembles pebble, stone, glass, diamond, etc.
Normally, the word "Phra Parama Sareerikathad" has the same meaning as "Buddha's relics". And the term "Phra Thad" means "the Relics". Even though the word "Relics" is not limited to just the the physical appearances, It usage is slighty different in Thai.