An SGHA is a Standard Ground Handling Agreement and covers all aspects of handling an aircraft on the ground including passengers, baggage, cargo, security, cleaning, etc. There are 14 sections to the agreement and the airlines can pick and choose which sections they want included and which they either want to do themselves or contract with someone else.
HZ123 A ramp agent covers baggage, a passenger agent covers passengers but they both work for a ground handling company.