Per workflow variant and release approval path, you determine which release approval procedure is triggered and carried out, and from which amount. These procedures are referred to as "subworkflows" in workflow terms.
The standard system comes with subworkflows predefined as workflow templates for use as a reference for your own subworkflows. The first of these contains a single-level release, the second, a two-level release and the third, a three-level release. The single-level release requires just one person to release the line items for payment. The two-level version supports dual control, whilst the three-level procedure allows for triple control of payment releases.
Assign the release approval procedure (subworkflow) that you require.