The -sX flag in Nmap performs a Xmas scan, which sends packets with the FIN, PSH, and URG flags set. This is an attempt to bypass firewalls and IDS/IPS devices by using a non-standard TCP packet. However, if the target port is closed, it will respond with a RST (reset) packet, indicating that there is no connection to be closed. This is how the penetration tester can infer that the ports in the target range are closed. If the port is open, the target will ignore the packet and not send any response. Reference:
* Nmap Cheat Sheet 2024: All the Commands & Flags - StationX
* Nmap Commands - 17 Basic Commands for Linux Network - phoenixNAP
* NMAP Flag Guide: What They Are, When to Use Them - CBT Nuggets
* [The Official CompTIA PenTest+ Self-Paced Study Guide (Exam PT0-002)], Chapter 4: Conducting Active Scanning, page 151.