Disable Contact timezone. What does it do? How does it affect the reminders?

This is important if you have clients from different timezones

If disabled contact timezone is Enable, the appointment reminders will not work properly and will not have the option to change on the contact side. (Location (Your) time zone compared to Prospect (Your Client) timezone)

The appointment reminders will reflect the location (your) timezone only and not the prospect's (your client) time zone.

Example:
A business in New York City wanted to book an appointment at 2PM EST from a company that is located in Los Angeles. The location (company in Los Angeles) had enable (disable contact timezone). So, they will see the appointment is 2PM PST and the business in New York City will get the appointment at 5PM ET.