A session rate charges per unit of time — typically per hour, half-day, or full day in the studio. This works well when the scope is uncertain or when the client needs flexibility.
A project rate is a flat fee for a defined deliverable — one mixed song, one mastered EP, one full production. This works best when the scope is clear and both parties agree on what 'done' means.
For clients: project rates give cost certainty. For providers: session rates protect against scope creep. Many professionals offer both options and advise based on the specific situation.