Hourly clients and retainer clients

Every client has a Billing setting that decides whether they can buy hours up front.

Retainer or hourly — they can buy plans and bundles. Their time draws down those hours, and any time the hours don't cover is billed hourly. This is the default for a new client.
Hourly only — they can't buy plans or bundles. All their time is billed at an hourly rate and invoiced.

Set it when you add a client, or change it any time from the client page: ⋯ → Edit details → Billing.

Give an hourly client a rate

An hourly client's time is priced by the client's Hourly rate (set in Edit details), unless a project or person rate applies. If you switch a client to Hourly only and they have no rate yet, the dialog reminds you to set one. See Set your rates.

What changes for an Hourly only client

On the client page:

No plans or bundles are offered: Sell a plan, adding hours and Sell hours are hidden.
The low-hours warning is hidden and never sent. Their stored threshold comes back if you switch them back.
The Overview shows Hourly only: billed at the client rate once nothing is left to wind down.
New projects for them default to Billed directly.

On the Clients list, their Hours left reads Hourly only, and you can filter the list with Billing → Hourly only.

In the client portal, they aren't offered hours to buy or plans to choose. See What your clients see in the portal.

Switching a client with hours or a plan

Switching either way cancels nothing and refunds nothing.

Hours they already hold stay theirs and keep drawing down until spent or lapsed.
A running plan keeps renewing. You can still cancel it, or call off a booked plan change, from the Overview, and the client can still cancel it from the portal. The plan itself can't be moved to a different plan while the client is Hourly only.
Switching back to Retainer or hourly gives them plans, bundles and the low-hours warning again.

To stop a plan when you switch a client to hourly, cancel it on the Overview tab. See Sell a retainer to a client.