When an agent logs into a campaign and their extension is 'Nailed Up' (see Agent Work Flow), the connection between their extension and the telephony layer is held open. This serves two purposes.
When logging an agent onto a campaign (including, for example, the System campaign to receive inbound calls), the configuration within Softdial Repository™ dictates whether the agent's extension is nailed up or not. See Static Configs for more details.
Agent nailup and login/ logout behaviour has been changed with V10.6.366. For more details, see Agent Work Flow.