What does the option * indicate?

The option 's' corresponds to a SYN scan (-sS in nmap), which sends half-open SYN packets to identify open ports and is not representative of a wildcard or flagless option.

The option 't' corresponds to a TCP connect scan (-sT in nmap), which completes the full three-way TCP handshake and is distinct from any null or flagless scan type.

In nmap, the -sN flag designates a NULL scan, which sends TCP packets with no control flags set. The 'n' stands for NULL, meaning the probe carries no SYN, ACK, RST, FIN, PSH, or URG bits. A NULL scan can evade stateless packet filters that only inspect specific flag combinations, making port state inference possible without triggering standard SYN-based firewall rules.

The option 'a' corresponds to an ACK scan (-sA in nmap), which is used to probe firewall rulesets rather than identify open ports, and does not represent a null or wildcard scan type.