The true coelom is categorized into two types- schizocoelom and enterocoelom. They are categorized based on ways in which true body cavity or coelom arises in different animals.
Schizocoelom arises from the mesodermal split. This body cavity is formed from blocks of mesoderm around the gut that enlarge and hollow out. Example- animals belonging to the Phyla Annelida, Mollusca, and Arthropoda.
Enterocoelom arises from outpocketing of the embryonic gut(endoderm). This type of body cavity is formed by outpocketings of the primitive gut that break off and form the coelom. Example- animals belonging to the Phyla Echinodermata and Chordata.
Hence Schlzocoelomates and enterocoelomates are true coelomates.