Is there an audit log?
Yes. The CRM records who changed what and when on an account from the day it opens, and reading the log is part of Business.
The audit log answers questions like who reassigned a lead, who deleted a contact, and whether a support session that signed in to the account changed anything.
Entries are written on every plan from the day an account opens, because history cannot be filled in afterwards: an office that upgrades to Business finds a log already there. Reading it is part of Business, since its purpose is managing other people.
Whoever may manage seats reads the whole account's log; everyone else sees only their own actions. Entries cannot be edited or deleted by anyone, including an admin.