The agency account is the top level of a GoHighLevel install. It sits above every sub-account and is where the agency creates and manages client sub-accounts, builds and shares snapshots, manages agency-level users, handles billing and rebilling, and sets white-label branding. HighLevel's API calls the agency a "company," so agency-level tokens and endpoints use companyId.
Agency users can be given access across all sub-accounts. Account users only see the sub-accounts they're assigned to.
In practice
When you log in as the agency owner, you land in the agency view. From there you switch into any client's sub-account, or stay at the agency level to work on things that span clients: creating a new sub-account, loading a snapshot into it, setting company-wide custom code in Settings under Company, or installing a marketplace app for many sub-accounts at once.
A common mistake is building something in the agency view and expecting it to appear in a sub-account. Custom fields, workflows and contacts always live in a sub-account. The agency level is for structure, access and branding.
Where it shows up in RGS Tools
RGS Tools is priced per agency, not per sub-account. Installing the marketplace app at the agency level connects every sub-account in one step, so each tool can list your sub-accounts in a dropdown. Power-Ups install with one loader line pasted into the agency's Custom JS setting, which makes them available in every sub-account. Backup & Insights's usage analytics summarizes activity per sub-account and per user across the agency.