For fixed locks this option is labelled “Unit of Time”. For variable locks it’s labelled as “Chance Regularity”.
For fixed locks you’re picking if you want the lock duration counted in steps of 15 minutes, hours or days. If you pick 15 minutes or hours you can create a lock that lasts between 15 minutes and 96 hours. These durations are good for testing the water for the first time, short self bondage sessions, or just a constant reminder that you’re locked. If you pick daily locks you can create a lock that lasts between 1-365 days. Fixed lock durations can’t be changed once it has been set up.
For variable locks you’re picking if you want a chance to reveal a card every 15 minutes, hour, or every day. Like fixed locks the 15 minutes and hourly option allows you to create a lock that lasts approximately 1-24 hours, and the daily option allows you to create a lock that lasts approximately 1-365+ days.