You can find the configuration for retainers in Leadtime within a project under Settings → General Billing Settings.
Retainers let you bill monthly hour packages at fixed flat rates.
They help you make recurring support, consulting, or maintenance services plannable and bill them automatically.
Unused hours can optionally be carried over to the following months.
To Projects – Settings – General Billing Settings

A retainer defines a monthly hour package that's available to the customer.
The package is suggested as a fixed price item in invoice review and is billed automatically once that month is due.
You can set the following parameters:
Retainer Title
Monthly flat rate
Hours per month
Internal reference hourly rate for calculating overtime
Option to carry over unused hours
Once activated, the retainer automatically affects cost calculation and hourly billing.
When checking the invoice, Leadtime differentiates between three billing scenarios:
If more hours are used than included, overtime is billed at the standard hourly rate.
If fewer hours are used and no carryover is enabled, the leftover volume expires.
If fewer hours are used and carryover is allowed, remaining hours roll over to the following months.

The resulting breakdown is displayed transparently in the invoice view.