You mean this file /var/db/captiveportalxxxxxxxx.db ?
This is SQLite3 file - it says so itself :
e2f234dd-8c71-464e-9d6d-33c449cfd7b0-image.png
As pfSense, you need this to interact with it.
Good news ; pfSense has the sqlite3 PHP extension loaded.
Friendly warning : the simple fact you had to this (IMHO : simple to find out yourself) means that you should not 'mess' with it.
Removing a 'record' in that database is can be done with these button button :
aa81f840-6474-4ec2-9c35-c79fa76a83d4-image.png
The other info stored in it (per record) is :
allow_time INTEGER,
pipeno INTEGER,
ip TEXT,
mac TEXT,
username TEXT,
sessionid TEXT,
bpassword TEXT,
session_timeout INTEGER,
idle_timeout INTEGER,
session_terminate_time INTEGER,
interim_interval INTEGER,
traffic_quota INTEGER,
bw_up INTEGER,
bw_down INTEGER,
authmethod TEXT,
context TEXT
Changing these values isn't useful, the underlying captive portal "ipfw" firewall rule won't get changed.