A managed tenancy is one for which you provide a fully-managed service ranging from finding an appropriate tenancy, through setting up the Assured Shorthold Tenancy (AST) agreement and collecting the security deposit, to collecting rent each month and taking care of maintenance jobs. For this service a management fee is charged, typically a percentage of rent collected but also possibly a flat fee per rental period, e.g. £50 per month.
A let-only tenancy is one which only requires the services of the letting agent at the outset to find a tenant, set them up in the property with a tenancy agreement and possibly collect the security deposit and first month's rent to pass to the landlord. Ongoing maintenance of the tenancy then becomes the landlord's responsibility.
Setting the tenancy type
When creating a tenancy the default type is Managed, so the Let-Only checkbox in the Rent and Charges section of the add tenancy form is unticked. To change this to Let-Only simply tick that checkbox.
If you set the tenancy to let-only, the Create and Apply Rent Schedule checkbox becomes unticked since that is the most common arrangement on let-only tenancies, but you can tick it again if required.
You can change the type of tenancy at any time via the tenancy menu, clicking Tenancy > Full Details > Update. This does not affect the way in which rent is applied though - read below.
Although you will typically only regularly apply rent on managed tenancies, you can set up one-off or recurring rent schedules on either type; it's up to you. It is common for agencies to collect the first month's rent on a let-only tenancy, which you can achieve by adding a one-off (non-recurring) rent charge.