A type descriptor is a record which holds information about a type. Slot 1 in the record must be a symbol naming the type, and type-of relies on this to return the type of record objects. No other type descriptor slot is used by Emacs; they are free for use by Lisp extensions.
An example of a type descriptor is any instance of cl-structure-class.
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https://www.gnu.org/software/emacs/manual/html_node/elisp/Type-Descriptors.html