The .nopayload field allows to scan packets which do not have payload.
Note This field cannot be used for signatures with template lookup and tag checking.
For example, this field can be used to find ports which have packets without payload. The signature for SYN scan detection is as follows:
UASL(.protocol=tcp; .tcp.flags=S; .rate=1000,2; .track=src_ip; .nopayload;)
Note Specifying the .nopayload field does not mean that the packets with payload are not scanned.