An inline set consists of paired Secure Firewall Threat Defense interfaces placed directly in the network traffic path. The interfaces operate like a bump in the wire: one interface receives the traffic, the inspection engine evaluates it, and the paired interface retransmits it unless an access-control, intrusion, file, or other security policy explicitly drops it. This arrangement enables the IPS to prevent attacks because the device can stop malicious packets before forwarding them. A passive interface also receives traffic unconditionally, but it receives copied traffic from a SPAN or mirror port and retransmits none of it. Routed and transparent modes describe broader firewall packet-forwarding behavior rather than this IPS-only interface characteristic.
Therefore, an inline deployment satisfies the requirement to inspect, retransmit, and selectively drop traffic.
Cisco inline-set documentation