A HPSG grammar includes principles and grammar rules and lexicon entries which are normally not considered to belong to a grammar.
The basic type HPSG deals with is the sign. It has two features PHON (the sound, the phonetic form) and the SYNSEM (syntactic and semantic) feature which is split into subfeatures subsequently.
Emily Bender, Ivan A. Sag, and Thomas Wasow Syntactic Theory: a formal introduction, Second Edition ([1])Books